BASIC INSTRUMENTATION
MEASURING DEVICES
AND
BASIC PID CONTROL
Science and Reactor Fundamentals – Instrumentation & Control
CNSC Technical Training Group
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Table of Contents
Section 1 - OBJECTIVES .................................................................... 3
Section 2 - INSTRUMENTATION EQUIPMENT ...................... 7
2.0
2.1
INTRODUCTION ......................................................................... 7
PRESSURE MEASUREMENT .................................................... 7
2.1.1 General Theory ................................................................... 7
2.1.2 Pressure Scales.................................................................... 7
2.1.3 Pressure Measurement ........................................................ 8
2.1.4 Common Pressure Detectors............................................... 9
2.1.5 Differential Pressure Transmitters .................................... 11
2.1.6 Strain Gauges .................................................................... 13
2.1.7 Capacitance Capsule ......................................................... 14
2.1.8 Impact of Operating Environment .................................... 15
2.1.9 Failures and Abnormalities ............................................... 16
2.2 FLOW MEASUREMENT........................................................... 18
2.2.1 Flow Detectors .................................................................. 18
2.2.2 Square Root Extractor....................................................... 25
2.2.3 Density Compensating Flow Detectors ............................ 29
2.2.4 Flow Measurement Errors................................................. 31
2.3 LEVEL MEASUREMENT ......................................................... 33
2.3.1 Level Measurement Basics ............................................... 33
2.3.2 Three Valve Manifold...................................................... 34
2.3.3 Open Tank Measurement.................................................. 36
2.3.4 Closed Tank Measurement ............................................... 36
2.3.5 Bubbler Level Measurement System ............................... 42
2.3.6 Effect of Temperature on Level Measurement ................. 44
2.3.7 Effect of Pressure on Level Measurement ....................... 47
2.3.8 Level Measurement System Errors.................................. 47
2.4 TEMPERATURE MEASUREMENT ......................................... 49
2.4.1 Resistance Temperature Detector (RTD)......................... 49
2.4.2 Thermocouple (T/C) ........................................................ 52
2.4.3 Thermal Wells.................................................................. 54
2.4.4 Thermostats......................................................................... 55
2.5 NEUTRON FLUX MEASUREMENT ....................................... 59
2.5.1 Neutron Flux Detection..................................................... 59
2.5.2 Neutron Detection Methods.............................................. 60
2.5.3 Start-up (sub-critical) Instrumentation............................. 61
2.5.4 Fission neutron detectors .................................................. 63
2.5.5 Ion chamber neutron detectors......................................... 64
2.5.6 In-Core Neutron Detectors............................................... 70
2.5.7 Reactor Control at High Power......................................... 77
2.5.8 Overlap of Neutron Detection........................................... 78
REVIEW QUESTIONS - EQUIPMENT ............................................. 81
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Section 3 - CONTROL ................................................................... 89
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
INTRODUCTION ....................................................................... 89
BASIC CONTROL PRINCIPLES .............................................. 89
3.1.1 Feedback Control .............................................................. 91
3.1.2 Feedforward Control........................................................ 91
3.1.3 Summary ........................................................................... 92
ON/OFF CONTROL ................................................................... 93
3.2.1 Summary ........................................................................... 94
BASIC PROPORTIONAL CONTROL ...................................... 95
3.3.1 Summary .......................................................................... 97
Proportional Control ................................................................... 98
3.4.1 Terminology..................................................................... 98
3.4.2 Practical Proportional Control ......................................... 98
3.4.3 Summary ......................................................................... 105
Reset of Integral Action ............................................................. 106
3.5.1 Summary ......................................................................... 109
RATE OR DERIVATIVE ACTION ........................................ 110
3.6.1 Summary ......................................................................... 115
MULTIPLE CONTROL MODES............................................. 116
TYPICAL NEGATIVE FEEDBACK CONTROL SCHEMES 117
3.8.1 Level Control .................................................................. 117
3.8.2 Flow Control ................................................................... 118
3.8.3 Pressure Control............................................................. 119
3.8.4 Temperature Control....................................................... 120
REVIEW QUESTIONS - CONTROL ...................................... 122
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OBJECTIVES
This module covers the following areas pertaining to instrumentation and
control.
•
•
•
•
•
•
Pressure
Flow
Level
Temperature
Neutron Flux
Control
At the end of training the participants will be able to:
Pressure
•
•
•
•
explain the basic working principle of pressure measuring devices,
bourdon tube, bellows, diaphragm, capsule, strain gauge,
capacitance capsule;
explain the basic operation of a differential pressure transmitter;
explain the effects of operating environment (pressure,
temperature, humidity) on pressure detectors;
state the effect of the following failures or abnormalities:
over-pressuring a differential pressure cell or bourdon tube;
diaphragm failure in a differential pressure cell;
blocked or leaking sensing lines; and
loss of loop electrical power.
Flow
• explain how devices generate a differential pressure signal: orifice,
venturi, flow nozzle, elbow, pitot tube, annubar;
• explain how each of the following will affect the indicated flow
signal from each of the above devices:
change in process fluid temperature;
change in process fluid pressure; and
erosion.
• identify the primary device, three-valve manifold and flow;
transmitter in a flow measurement installation;
• state the relationship between fluid flow and output signal in a
flow control loop with a square root extractor;
• describe the operation of density compensating flow detectors;
• explain why density compensation is required in some flow
measurements;
• state the effect on the flow measurement in process with
abnormalities: Vapour formation in the throat, clogging if throat by
foreign material, Leaks in HI or LO pressure sensing lines;
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Level
• explain how a level signal is derived for: an open vessel, a
closed vessel with dry reference leg, a closed vessel with wet
reference leg;
• explain how a DP cell can be damaged from over pressure if it
is not isolated correctly;
• explain how a bubbler derives level signal for an open and
closed tank;
• explain the need for zero suppression and zero elevation in level
measurement installations;
• describe the effects of varying liquid temperature or pressure on
level indication from a differential pressure transmitter;
• explain how errors are introduced into the DP cell signal by
abnormalities: leaking sensing lines, dirt or debris in the sensing
lines;
Temperature
• explain the principle of operation of temperature detectors: RTD,
thermocouple, bimetallic strip & pressure cylinders;
• state the advantages and disadvantages of RTDs and
thermocouples
• state the effect on the indicated temperature for failures, open
circuit and short circuit;
Flux
• state the reactor power control range for different neutron sensors
and explain why overlap is required: Start-up instrumentation, Ion
Chambers, In Core detectors;
• explain how a neutron flux signal is derived in a BF3 proportional
counter;
• explain the reasons for start-up instrumentation burn-out;
• explain how a neutron flux signal is derived in an ion chamber;
• state the basic principles of operation of a fission chamber
radiation detector;
• state and explain methods of gamma discrimination for neutron ion
chambers;
• explain how the external factors affect the accuracy of the ion
chamber’s neutron flux measurement: Low moderator level, Loss
of high voltage power supply, Shutdown of the reactor;
• describe the construction and explain the basic operating principle
of in-core neutron detectors;
• explain reactor conditions factors can affect the accuracy of the incore detector neutron flux measurement: Fuelling or reactivity
device movement nearby, Start-up of the reactor, long-term
exposure to neutron flux, Moderator poison (shielding);
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explain the reasons for power control using ion chambers at low
power and in-core detectors at high power;
Control
• identify the controlled and manipulated variables;
• sketch a simple block diagram and indicate set point,
measurement, error, output and disturbances;
• state the difference between open and closed loop control;
• state the basic differences between feedback and feed forward
control;
• explain the general on/off control operation;
• explain why a process under on/off control is not controllable at
the set point;
• explain why on/off control is suitable for slow responding
processes;
• explain the meaning of proportional control in terms of the
relationship between the error signal and the control signal;
• explain why offset will occur in a control system, with
proportional control only;
• choose the controller action for corrective control;
• convert values of PB in percentage to gain values and vice-versa;
• determine the relative magnitude of offset with respect to the
proportional band setting;
• state the accepted system response, i.e., ¼ decay curve, following a
disturbance;
• explain the reason for the use of reset (integral) control and its
units;
• sketch the open loop response curve for proportional plus reset
control in response to a step disturbance;
• state two general disadvantages of reset control with respect to
overall loop stability and loop response if the control setting is
incorrectly adjusted;
• calculate the reset action in MPR or RPM given a control system’s
parameters;
• state, the purpose of rate or derivative control;
• state the units of derivative control;
• justify the use of rate control on slow responding processes such
as heat exchangers;
• explain why rate control is not used on fast responding
processes.
• sketch the open loop response curve for a control system with
proportional plus derivative control modes;
• state which combinations of the control modes will most likely
be found in typical control schemes;
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sketch typical control schemes for level, pressure, flow and
temperature applications.
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INSTRUMENTATION EQUIPMENT
2.0
INTRODUCTION
Instrumentation is the art of measuring the value of some plant parameter,
pressure, flow, level or temperature to name a few and supplying a signal
that is proportional to the measured parameter. The output signals are
standard signal and can then be processed by other equipment to provide
indication, alarms or automatic control. There are a number of standard
signals; however, those most common in a CANDU plant are the 4-20 mA
electronic signal and the 20-100 kPa pneumatic signal.
This section of the course is going to deal with the instrumentation
equipment normal used to measure and provide signals. We will look at
the measurement of five parameters: pressure, flow, level, temperature,
and neutron flux.
2.1
PRESSURE MEASUREMENT
This module will examine the theory and operation of pressure detectors
(bourdon tubes, diaphragms, bellows, forced balance and variable
capacitance). It also covers the variables of an operating environment
(pressure, temperature) and the possible modes of failure.
2.1.1 General Theory
Pressure is probably one of the most commonly measured variables in the
power plant. It includes the measurement of steam pressure; feed water
pressure, condenser pressure, lubricating oil pressure and many more.
Pressure is actually the measurement of force acting on area of surface.
We could represent this as:
Pressure
Force
Area
or
P
F
A
The units of measurement are either in pounds per square inch (PSI) in
British units or Pascals (Pa) in metric. As one PSI is approximately 7000
Pa, we often use kPa and MPa as units of pressure.
2.1.2 Pressure Scales
Before we go into how pressure is sensed and measured, we have to
establish a set of ground rules. Pressure varies depending on altitude above
sea level, weather pressure fronts and other conditions.
The measure of pressure is, therefore, relative and pressure measurements
are stated as either gauge or absolute.
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Gauge pressure is the unit we encounter in everyday work (e.g., tire
ratings are in gauge pressure).
A gauge pressure device will indicate zero pressure when bled down to
atmospheric pressure (i.e., gauge pressure is referenced to atmospheric
pressure). Gauge pressure is denoted by a (g) at the end of the pressure
unit [e.g., kPa (g)].
Absolute pressure includes the effect of atmospheric pressure with the
gauge pressure. It is denoted by an (a) at the end of the pressure unit [e.g.,
kPa (a)]. An absolute pressure indicator would indicate atmospheric
pressure when completely vented down to atmosphere - it would not
indicate scale zero.
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
Figure 1 illustrates the relationship between absolute and gauge. Note
that the base point for gauge scale is [0 kPa (g)] or standard atmospheric
pressure 101.3 kPa (a).
The majority of pressure measurements in a plant are gauge. Absolute
measurements tend to be used where pressures are below atmosphere.
Typically this is around the condenser and vacuum building.
Absolute
Scale
Atmospheric
Pressure
Perfect
Vacuum
Gauge
Scale
101.3 kPa(a)
0 kPa(g)
0 kPa(a)
-101.3 kPa(g)
Figure 1
Relationship between Absolute and Gauge Pressures
2.1.3 Pressure Measurement
The object of pressure sensing is to produce a dial indication, control
operation or a standard (4 - 20 mA) electronic signal that represents the
pressure in a process.
To accomplish this, most pressure sensors translate pressure into physical
motion that is in proportion to the applied pressure. The most common
pressure sensors or primary pressure elements are described below.
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They include diaphragms, pressure bellows, bourdon tubes and pressure
capsules. With these pressure sensors, physical motion is proportional to
the applied pressure within the operating range.
You will notice that the term differential pressure is often used. This term
refers to the difference in pressure between two quantities, systems or
devices
2.1.4
Common Pressure Detectors
Bourdon Tubes
Bourdon tubes are circular-shaped tubes with oval cross sections (refer to
Figure 2). The pressure of the medium acts on the inside of the tube. The
outward pressure on the oval cross section forces it to become rounded.
Because of the curvature of the tube ring, the bourdon tube then bends as
indicated in the direction of the arrow.
Motion
Cross
Section
Pressure
Figure 2
Bourdon Tube
Due to their robust construction, bourdon are often used in harsh
environments and high pressures, but can also be used for very low
pressures; the response time however, is slower than the bellows or
diaphragm.
Bellows
Bellows type elements are constructed of tubular membranes that are
convoluted around the circumference (see Figure 3). The membrane is
attached at one end to the source and at the other end to an indicating
device or instrument. The bellows element can provide a long range of
motion (stroke) in the direction of the arrow when input pressure is
applied.
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Motion
Note
Flexible
Bellows
Pressure
Figure 3
Bellows
Diaphragms
A diaphragm is a circular-shaped convoluted membrane that is attached to
the pressure fixture around the circumference (refer to Figure 4). The
pressure medium is on one side and the indication medium is on the other.
The deflection that is created by pressure in the vessel would be in the
direction of the arrow indicated.
Motion
Flexible
Membrane
.
Pressure
Figure 4
Diaphragm
Diaphragms provide fast acting and accurate pressure indication.
However, the movement or stroke is not as large as the bellows
Capsules
There are two different devices that are referred to as capsule. The first is
shown in figure 5. The pressure is applied to the inside of the capsule and
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if it is fixed only at the air inlet it can expand like a balloon. This
arrangement is not much different from the diaphragm except that it
expands both ways.
Motion
Flexible
Membranes
continuous
seam
seam
Pressure
Figure 5
Capsule
The capsule consists of two circular shaped, convoluted membranes
(usually stainless steel) sealed tight around the circumference. The
pressure acts on the inside of the capsule and the generated stroke
movement is shown by the direction of the arrow.
The second type of capsule is like the one shown in the differential
pressure transmitter (DP transmitter) in figure 7. The capsule in the bottom
is constructed with two diaphragms forming an outer case and the interspace is filled with viscous oil. Pressure is applied to both side of the
diaphragm and it will deflect towards the lower pressure.
To provide over-pressurized protection, a solid plate with diaphragmmatching convolutions is usually mounted in the center of the capsule.
Silicone oil is then used to fill the cavity between the diaphragms for even
pressure transmission.
Most DP capsules can withstand high static pressure of up to 14 MPa
(2000 psi) on both sides of the capsule without any damaging effect.
However, the sensitive range for most DP capsules is quite low. Typically,
they are sensitive up to only a few hundred kPa of differential pressure.
Differential pressure that is significantly higher than the capsule range
may damage the capsule permanently.
2.1.5 Differential Pressure Transmitters
Most pressure transmitters are built around the pressure capsule concept.
They are usually capable of measuring differential pressure (that is, the
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difference between a high pressure input and a low pressure input) and
therefore, are usually called DP transmitters or DP cells.
Figure 6 illustrates a typical DP transmitter. A differential pressure
capsule is mounted inside a housing. One end of a force bar is connected
to the capsule assembly so that the motion of the capsule can be
transmitted to outside the housing. A sealing mechanism is used where the
force bar penetrates the housing and also acts as the pivot point for the
force bar. Provision is made in the housing for high- pressure fluid to be
applied on one side of the capsule and low-pressure fluid on the other.
Any difference in pressure will cause the capsule to deflect and create
motion in the force bar. The top end of the force bar is then connected to a
position detector, which via an electronic system will produce a 4 - 20 ma
signal that is proportional to the force bar movement.
4-20mA
Detector
Seal and Pivot
Force Bar
Silicone Oil Filling
High Pressure
Low Pressure
D.P. Capsule
Housing
Metal Diaphragm
Backup Plate
Figure 6
Typical DP Transmitter Construction
This DP transmitter would be used in an installation as shown in
Figure 7.
Pressure Transmitter
Controlled Vessel
Pressure
(20 to 30 KPa)
Impulse Line
Isolation
Valve
HL
Vented
4-20mA
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Figure 7
DP Transmitter Application
Note
A DP transmitter is used to measure the gas pressure (in gauge scale)
inside a vessel. In this case, the low-pressure side of the transmitter is
vented to atmosphere and the high-pressure side is connected to the vessel
through an isolating valve. The isolating valve facilitates the removal of
the transmitter.
The output of the DP transmitter is proportional to the gauge pressure of
the gas, i.e., 4 mA when pressure is 20 kPa and 20 mA when pressure is
30 kPa.
2.1.6 Strain Gauges
The strain gauge is a device that can be affixed to the surface of an object
to detect the force applied to the object. One form of the strain gauge is a
metal wire of very small diameter that is attached to the surface of a
device being monitored.
Resistance Ω Increases
Length Increases
Force
AREA
AREA
AREA
Force
Cross Sectional Area Decreases
Figure 8
Strain Gauge
For a metal, the electrical resistance will increase as the length of the
metal increases or as the cross sectional diameter decreases.
When force is applied as indicated in Figure 8, the overall length of the
wire tends to increase while the cross-sectional area decreases.
The amount of increase in resistance is proportional to the force that
produced the change in length and area. The output of the strain gauge is a
change in resistance that can be measured by the input circuit of an
amplifier.
Strain gauges can be bonded to the surface of a pressure capsule or to a
force bar positioned by the measuring element. Shown in Figure 9 (next
page) is a strain gauge that is bonded to a force beam inside the DP
capsule. The change in the process pressure will cause a resistive change
in the strain gauges, which is then used to produce a 4-20 mA signal.
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Electronics Enclosure
14
Field Terminals
Note
Electronic
Amplifier
Compensation
Circuit Board
Electronics
Feedthrough
Liquid Fill
Process Seal
Diaphragm
Strain Gauge
Beam
Overpressure
Stop
Sensing Capsular
Element
Figure 9
Resistive Pressure Transmitter
2.1.7 Capacitance Capsule
Similar to the strain gauge, a capacitance cell measures changes in
electrical characteristic. As the name implies the capacitance cell measures
changes in capacitance. The capacitor is a device that stores electrical
charge. It consists of metal plates separated by an electrical insulator. The
metal plates are connected to an external electrical circuit through which
electrical charge can be transferred from one metal plate to the other.
The capacitance of a capacitor is a measure of its ability to store charge.
The capacitance of the capacitance of a capacitor is directly proportional
to the area of the metal plates and inversely proportional to the distance
between them. It also depends on a characteristic of the insulating material
between them. This characteristic, called permittivity is a measure of how
well the insulating material increases the ability of the capacitor to store
charge.
C =ε
A
d
C is the capacitance in Farads
A is the area of the plates
D is the distance of the plates
ε is the permittivity of the insulator
By building a DP cell capsule so there are capacitors inside the cell
capsule, differential pressures can be sensed by the changes in capacitance
of the capacitors as the pressure across the cell is varied.
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2.1.8 Impact of Operating Environment
All of the sensors described in this module are widely used in control and
instrumentation systems throughout the power station.
Their existence will not normally be evident because the physical
construction will be enclosed inside manufacturers’ packaging. However,
each is highly accurate when used to measure the right quantity and within
the rating of the device. The constraints are not limited to operating
pressure. Other factors include temperature, vapour content and vibration.
Vibration
The effect of vibration is obvious in the inconsistency of measurements,
but the more dangerous result is the stress on the sensitive membranes,
diaphragms and linkages that can cause the sensor to fail. Vibration can
come from many sources.
Some of the most common are the low level constant vibration of an
unbalanced pump impeller and the larger effects of steam hammer.
External vibration (loose support brackets and insecure mounting) can
have the same effect.
Temperature
The temperature effects on pressure sensing will occur in two main areas:
The volumetric expansion of vapour is of course temperature dependent.
Depending on the system, the increased pressure exerted is usually already
factored in.
The second effect of temperature is not so apparent. An operating
temperature outside the rating of the sensor will create significant error in
the readings. The bourdon tube will indicate a higher reading when
exposed to higher temperatures and lower readings when abnormally cold
- due to the strength and elasticity of the metal tube. This same effect
applies to the other forms of sensors listed.
Vapour Content
The content of the gas or fluid is usually controlled and known. However,
it is mentioned at this point because the purity of the substance whose
pressure is being monitored is of importance - whether gaseous or fluid –
especially, if the device is used as a differential pressure device in
measuring flow of a gas or fluid.
Higher than normal density can force a higher dynamic reading depending
on where the sensors are located and how they are used. Also, the vapour
density or ambient air density can affect the static pressure sensor readings
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and DP cell readings. Usually, lower readings are a result of the lower
available pressure of the substance. However, a DP sensor located in a hot
and very humid room will tend to read high.
2.1.9
Failures and Abnormalities
Over-Pressure
All of the pressure sensors we have analyzed are designed to operate over
a rated pressure range. Plant operating systems rely on these pressure
sensors to maintain high accuracy over that given range. Instrument
readings and control functions derived from these devices could place
plant operations in jeopardy if the equipment is subjected to over pressure
(over range) and subsequently damaged. If a pressure sensor is over
ranged, pressure is applied to the point where it can no longer return to its
original shape, thus the indication would return to some value greater than
the original.
Diaphragms and bellows are usually the most sensitive and fast-acting of
all pressure sensors.
They are also however, the most prone to fracture on over-pressuring.
Even a small fracture will cause them to read low and be less responsive to
pressure changes. Also, the linkages and internal movements of the
sensors often become distorted and can leave a permanent offset in the
measurement. Bourdon tubes are very robust and can handle extremely
high pressures although, when exposed to over-pressure, they become
slightly distended and will read high. Very high over-pressuring will of
course rupture the tube.
Faulty Sensing Lines
Faulty sensing lines create inaccurate readings and totally misrepresent the
actual pressure
When the pressure lines become partially blocked, the dynamic response
of the sensor is naturally reduced and it will have a slow response to
change in pressure. Depending on the severity of the blockage, the sensor
could even retain an incorrect zero or low reading, long after the change in
vessel pressure.
A cracked or punctured sensing line has the characteristic of consistently
low readings. Sometimes, there can be detectable down-swings of pressure
followed by slow increases.
Loss of Loop Electrical Power
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As with any instrument that relies on AC power, the output of the D/P
transmitters will drop to zero or become irrational with a loss of power
supply.
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2.2
18
FLOW MEASUREMENT
There are various methods used to measure the flow rate of steam, water,
lubricants, air, etc., in a nuclear generating station. However, in this module
will look at the most common, namely the DP cell type flow detector. Also
in this section we will discuss the application of a square root extractor and
cut-off relay plus the possible sources of errors in flow measurements and
different failure modes that can occur.
2.2.1 Flow Detectors
To measure the rate of flow by the differential pressure method, some form
of restriction is placed in the pipeline to create a pressure drop. Since flow in
the pipe must pass through a reduced area, the pressure before the restriction
is higher than after or downstream. Such a reduction in pressure will cause
an increase in the fluid velocity because the same amount of flow must take
place before the restriction as after it. Velocity will vary directly with the
flow and as the flow increases a greater pressure differential will occur
across the restriction. So by measuring the differential pressure across a
restriction, one can measure the rate of flow.
Orifice Plate
The orifice plate is the most common form of restriction that is used in flow
measurement. An orifice plate is basically a thin metal plate with a hole
bored in the center. It has a tab on one side where the specification of the
plate is stamped. The upstream side of the orifice plate usually has a sharp,
edge. Figure 1 shows a representative orifice plate.
Orifice Plate
Flow
Sharp Edge
High Pressure
Sensing Line
Bevel
Low Pressure
Sensing Line
Figure 1
A Typical Orifice Plate
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When an orifice plate is installed in a flow line (usually clamped between a
pair of flanges), increase of fluid flow velocity through the reduced area at
the orifice develops a differential pressure across the orifice. This pressure is
a function of flow rate.
Pressure Change
With an orifice plate in the pipe work, static pressure increases slightly
upstream of the orifice (due to back pressure effect) and then decreases
sharply as the flow passes through the orifice, reaching a minimum at a
point called the vena contracta where the velocity of the flow is at a
maximum. Beyond this point, static pressure starts to recover as the flow
slows down. However, with an orifice plate, static pressure downstream is
always considerably lower than the upstream pressure. In addition some
pressure energy is converted to sound and heat due to friction and
turbulence at the orifice plate. Figure 2 shows the pressure profile of an
orifice plate installation.
Flanges
Permanent Pressure Loss
Vena Contacts
Orifice Plate
Figure 2
Orifice Plate Installation with Pressure Profile
On observing Figure 2, one can see that the measured differential pressure
developed by an orifice plate also depends on the location of the pressure
sensing points or pressure taps.
Flange Taps
Flange taps are the most widely used pressure tapping location for orifices.
They are holes bored through the flanges, located one inch upstream and one
inch downstream from the respective faces of the orifice plate. A typical
flange tap installation is shown in Figure 3. The upstream and downstream
sides of the orifice plate are connected to the high pressure and low-pressure
sides of a DP transmitter. A pressure transmitter, when installed to measure
flow, can be called a flow transmitter. As in the case of level measurement,
the static pressure in the pipe-work could be many times higher than the
differential pressure created by the orifice plate.
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In order to use a capsule that is sensitive to low differential pressure, a threevalve manifoldhas to be used to protect the DP capsule from being overranged. The three valve manifold is discussed in more detail in the section
on level measurement.
Flow
1"
H.P. Isolating Valve
L.P. Isolating Valve
Equalizer Valve
H
L
H.P. Block
L.P. Block
FT
D/P Cell
Figure 3
Orifice Plate with Flange Taps and Three Valve Manifold
Corner Taps
Corner taps are located right at upstream and downstream faces of the
orifice plates (see Figure 4).
Flow
H.P.
L.P.
Figure 4
Orifice Plate with Corner Taps
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Vena Contracta Taps
Vena contracta taps are located one pipe inner diameter upstream and at the
point of minimum pressure, usually one half pipe inner diameter
downstream (Figure 5).
Flow
H.P.
1D
Usually
1/2 D
L.P.
Figure 5
Orifice Plate with Vena Contracta Taps
Pipe Taps
Pipe taps are located two and a half pipe inner diameters upstream and eight
pipe inner diameters downstream.
When an orifice plate is used with one of the standardized pressure tap
locations, an on-location calibration of the flow transmitter is not necessary.
Once the ratio and the kind of pressure tap to be used are decided, there are
empirically derived charts and tables available to facilitate calibration.
Advantages and Disadvantages of Orifice Plates
Advantages of orifice plates include:
•
High differential pressure generated
•
Exhaustive data available
•
Low purchase price and installation cost
•
Easy replacement
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Disadvantages include:
•
High permanent pressure loss implies higher pumping cost.
•
Cannot be used on dirty fluids, slurries or wet steam as erosion will
alter the differential pressure generated by the orifice plate.
Venturi Tubes
For applications where high permanent pressure loss is not tolerable, a
venturi tube (Figure 6) can be used. Because of its gradually curved inlet
and outlet cones, almost no permanent pressure drop occurs. This design
also minimizes wear and plugging by allowing the flow to sweep suspended
solids through without obstruction.
H.P.
L.P.
Flow
Figure 6
Venturi Tube Installation
However a Venturi tube does have disadvantages:
•
Calculated calibration figures are less accurate than for orifice plates.
For greater accuracy, each individual Venturi tube has to be flow
calibrated by passing known flows through the Venturi and
recording the resulting differential pressures.
•
The differential pressure generated by a venturi tube is lower than
for an orifice plate and, therefore, a high sensitivity flow transmitter
is needed.
•
It is more bulky and more expensive.
As a side note; one application of the Venturi tube is the measurement of
flow in the primary heat transport system. Together with the temperature
change across these fuel channels, thermal power of the reactor can be
calculated.
Flow Nozzle
A flow nozzle is also called a half venturi. Figure 7 shows a typical flow
nozzle installation.
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23
.5 D
Note
H.P.
L.P.
Flow
D
Flow Nozzle
Figure 7
Flow Nozzle Installation
The flow nozzle has properties between an orifice plate and a venturi.
Because of its streamlined contour, the flow nozzle has a lower permanent
pressure loss than an orifice plate (but higher than a venturi). The
differential it generates is also lower than an orifice plate (but again higher
than the venturi tube). They are also less expensive than the venturi tubes.
Flow nozzles are widely used for flow measurements at high velocities.
They are more rugged and more resistant to erosion than the sharp-edged
orifice plate. An example use of flow nozzles are the measurement of flow
in the feed and bleed lines of the PHT system.
Elbow Taps
Centrifugal force generated by a fluid flowing through an elbow can be used
to measure fluid flow. As fluid goes around an elbow, a high-pressure area
appears on the outer face of the elbow. If a flow transmitter is used to sense
this high pressure and the lower pressure at the inner face of the elbow, flow
rate can be measured. Figure 8 shows an example of an elbow tap
installation.
One use of elbow taps is the measurement of steam flow from the boilers,
where the large volume of saturated steam at high pressure and temperature
could cause an erosion problem for other primary devices.
Another advantage is that the elbows are often already in the regular piping
configuration so no additional pressure loss is introduced.
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Note
H.P.
L.P.
45˚
Flow
Figure 8
Elbow Tap Installation
Pitot Tubes
Pitot tubes also utilize the principles captured in Bernoulli’s equation, to
measure flow. Most pitot tubes actually consist of two tubes. One, the lowpressure tube measures the static pressure in the pipe. The second, the highpressure tube is inserted in the pipe in such a way that the flowing fluid is
stopped in the tube. The pressure in the high-pressure tube will be the static
pressure in the system plus a pressure dependant on the force required
stopping the flow.
Figure 9
Pitot Tube
Pitot tubes are more common measuring gas flows that liquid flows. They
suffer from a couple of problems.
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The pressure differential is usually small and hard to measure.
The differing flow velocities across the pipe make the accuracy dependent
on the flow profile of the fluid and the position of the pitot in the pipe.
Annubar
An annubar is very similar to a pitot tube. The difference is that there is
more than one hole into the pressure measuring chambers. The pressure in
the high-pressure chamber represents an average of the velocity across the
pipe. Annubars are more accurate than pitots as they are not as position
sensitive or as sensitive to the velocity profile of the fluid.
Figure 10
Annubar
2.2.2 Square Root Extractor
Up to now, our flow measurement loop can be represented by the
installation shown in Figure 9. The high and low-pressure taps of the
primary device (orifice type shown) are fed by sensing lines to a differential
pressure (D/P) cell. The output of the D/P cell acts on a pressure to milliamp transducer, which transmits a variable 4-20 ma signal. The D/P cell and
transmitter are shown together as a flow transmitter (FT).
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Orifice Plate
Note
Flow
High
Pressure
Low
Pressure
FT
4-20mA
∆P
Figure 11
A Flow Loop with Orifice Plate
This simple system although giving an indication of the flow rate (Q), is
actually transmitting a signal proportional to the differential pressure (∆P).
However, the relationship between the volume of flow Q and ∆P is not
linear. Thus such a system would not be appropriate in instrumentation or
metering that requires a linear relationship or scale.
In actuality the differential pressure increases in proportion to the square of
the flow rate.
We can write this as: ∆P ∝ Q 2
In other words the flow rate (Q) is proportional; to the square root of the
differential pressure.
Volumetric Flow Rate = Q ∝
∆P
To convert the signal from the flow transmitter, (figure 9 above) to one that
is directly proportional to the flow-rate, one has to obtain or extract the
square root of the signal from the flow transmitter. Figure 10 illustrates the
input - output relationship of a square root extractor.
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Output
100%
(20mA)
Note
86.6%
(17.86mA)
70.7%
(15.3mA)
50%
(12mA)
0%
(4mA)
25%
(8mA)
50%
(12mA)
75%
(16mA)
100%
(20mA)
Input
from FT
Figure 12
Square Root Extractor Input and Output
The square root extractor is an electronic (or pneumatic) device that takes
the square root of the signal from the flow transmitter and outputs a
corresponding linear flow signal. Several methods are used in the
construction of square root extractors. However, it is beyond the scope of
this course to discuss the actual circuitries.
A typical square root extractor installation is shown in Figure 13. This
system would produce a 4-20-ma signal that is linear with the flow rate.
Orifice Plate
Flow
High
Pressure
Low
Pressure
FT
4-20mA
∆P
4-20mA
Q
Controller
Figure 13
A Typical Square Root Extractor Installation
Square root extractors are usually current operated devices so they can be
connected directly in the 4-20 mA current loop of a flow transmitter. The
output of the square root extractor is again a 4-20 mA signal. This signal is
directly proportional to the flow-rate in the pipe-work.
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The signal from the square root extractor usually goes to a controller, as
shown in Figure 13.
The controller (which can be regarded as an analog computer) is used to
control the final control element, usually a valve.
b
B
A
a
Output =
Input
Cut-off relay
Square root extractors do have a drawback. At low values of input, very
small changes in the input (differential pressure) to the extractor will cause a
large change in the square root output (flow indication). This system is
described as having high gain at values close to zero input. Observe figure
14 below, which is an expanded version of figure 12 at the lower end. The
amount of change from zero pressure to A and from A to B is identical.
However, for the same input change (∆P), the gain at low input is greater.
0
a
Square Root Extractor
b
Input % Change
Figure 14
Square Root Extractor Graph Expanded View
To illustrate the effect of the very high gain in the square root extractor at
low scale values consider a typical situation. A pipe valve is closed and the
zero flow produces a 4 mA output from the flow transmitter. If due to noise,
temperature or other disturbances, the input drifted from 0% to 1% (i.e.,
from 4 mA to 4.16 mA), the output would have changed from 0% to 10% (4
mA to 5.6 mA). It is obvious that this significant error sent to the controller
has to be eliminated.
For this reason, square root extractors are equipped with cut-off relays. The
setting for the relay can be adjusted from 6% to 10% of output. Shown in
Figure 15 is a response curve for a cut-off relay set at 7% output. In this
case, any input signal below (0.07)2 or 0.49% would be ignored by the
extractor. The output of the extractor would remain at 0% as long as input is
below 0.49%.
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Output - Percent
When the input exceeded 0.49%, the output would resume its normal curve,
starting at 7%.
10
9
8
7
6
5
4
3
2
1
Cutoff
Point
Square Root Curve
(Low end)
0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0
Input - Percent
Figure 15
Response Curve for Extractor with 7% Cut-Off Setting
2.2.3 Density Compensating Flow Detectors
It must be remembered that a DP transmitter used for flow measurement,
measures differential pressure, not the volume or mass of flow. We have
shown that differential pressure instruments require that the square root
differential pressure be taken to obtain volumetric flow Q:
Volume of Flow = Q ∝ ∆P / ρ
For compressible vapour such as steam, it is more important to know the
mass of the flow W rather than the volume. To determine the mass of a
liquid/gas the density (ρ = mass per unit volume) must also be obtained.
Mass of Flow = W = ρQ ∝ ρ∆P
We also know that density varies directly with pressure and inversely with
temperature:
ρ α K
pressure
temperature
The coefficient K (which can be obtained from tables) depends on a number
of variables including the pipe size and the characteristics of the fluid/gas. It
is sufficient to say that if the process temperature and static pressure is
known, then the density can be obtained.
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Note
Flow
High Pressure
Sensing Line
RTD
Low
Pressure
Sensing Line
Pressure
Cell
DP Cell
Logic
4-20 mA Output
Figure 16
Density Compensating Flow Detector
The density compensating flow detector (shown schematically in
figure 16) is a necessity for steam flow between the boilers, re-heaters and
the turbines, where the mass (weight) of the steam is more important than
the volume.
Process Conditions
As previously stated, the measurement of flow using any of the devices
described above is purely inferential. It relies on the signal from a
differential pressure (D/P) cell to obtain an inferred flow measurement. This
flow measurement could be either the volume or mass of the liquid/gas. In
either case the instrumentation can be affected by the process conditions.
The three main parameters are:
Fluid Temperature
The temperature of the flow quantity has a dramatic effect on the flow
measurement. Under the right conditions the liquid can either boil
(producing gas pockets and turbulence) or freeze (producing blockages and
distorted flow patterns) at the sensors.
At the onset of temperature related flow instrumentation problems the meter
readings will become unstable. Gas pockets (causing intermittent low
pressure) at the high pressure sensing lines will cause apparent low flow
fluctuations. This is more predominant in orifice and flow-nozzle
installations. Turbulence at the low-pressure sensor will usually increase as
the temperature increases to cause a more stable but incorrect high flow
reading.
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Temperature also affects the density of the liquid/gas, as per the following
relationship (where K is a constant for the liquid/gas).
ρ α K
pressure
temperature
The mass flow (i.e., pounds of steam per minute) varies inversely with
temperature and must be compensated for using a density compensating
flow detector.
The elbow tap sensor uses centrifugal force to detect flow and is most
sensitive to density changes. The flow readings will increase as the
temperature decreases.
Fluid Pressure
As we have just seen, pressure also affects the density of the fluid/gas. For
the elbow tap previously mentioned, the flow readings will increase as the
process pressure increases.
ρ α K
pressure
temperature
For all types of D/P flow sensors, mass flow will of course increase as the
pressure increases. To obtain the correct measurement of mass flow, a
density compensating flow detector must be used as described previously.
2.2.4 Flow Measurement Errors
We have already discussed the pros and cons of each type of flow detector
commonly found in a generating station. Some, such as the orifice, are more
prone to damage by particulate or saturated steam then others. However,
there are common areas where the flow readings can be inaccurate or
invalid.
Erosion
Particulate, suspended solids or debris in the piping will not only plug up the
sensing lines, it will erode the sensing device. The orifice, by its design with
a thin, sharp edge is most affected, but the flow nozzle and even venturi can
also be damaged. As the material wears away, the differential pressure
between the high and low sides of the sensor will drop and the flow reading
will decrease.
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Over ranging Damage to the D/P Cell
Again, as previously described, the system pressures are usually much
greater than the differential pressure and three valve manifolds must be
correctly used.
Vapour Formation in the Throat
D/P flow sensors operate on the relation between velocity and pressure. As
gas requires less pressure to compress, there is a greater pressure differential
across the D/P cell when the gas expands on the LP side of the sensor. The
flow sensor will indicate a higher flow rate than there actually is. The
turbulence created at the LP side of the sensor will also make the reading
somewhat unstable. A small amount of gas or vapour will make a large
difference in the indicated flow rate.
The opposite can occur if the vapour forms in the HP side of the sensor due
to cavitation or gas pockets when the fluid approaches the boiling point. In
such an instance there will be a fluctuating pressure drop across the D/P cell
that will give an erroneously low (or even negative) D/P reading.
Clogging of Throat
Particulate or suspended solids can damage the flow sensor by the high
velocities wearing at the flow sensor surfaces. Also, the build-up of material
in the throat of the sensor increases the differential pressure across the cell.
The error in flow measurement will increase as the flow increases.
Plugged or Leaking Sensing Lines
The effects of plugged or leaking D/P sensing lines is the same as described
in previous modules, however the effects are more pronounced with the
possible low differential pressures. Periodic maintenance and bleeding of
the sensing lines is a must. The instrument error will depend on where the
plug/leak is:
On the HP side a plugged or leaking sensing line will cause a lower reading.
The reading will become irrational if the LP pressure equals or exceeds the
HP sensing pressure.
On the LP side a plugged or leaking sensing line will cause a higher reading.
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2.3 LEVEL MEASUREMENT
Accurate continuous measurement of volume of fluid in containers has
always been a challenge to industry. This is even more so in the nuclear
station environment where the fluid could be acidic/caustic or under very
high pressure/temperature. We will now examine the measurement of fluid
level in vessels and the effect of temperature and pressure on this
measurement. We will also consider the operating environment on the
measurement and the possible modes of device failure.
2.3.1 Level Measurement Basics
Very simple systems employ external sight glasses or tubes to view the
height and hence the volume of the fluid. Others utilize floats connected to
variable potentiometers or rheostats that will change the resistance
according to the amount of motion of the float. This signal is then inputted
to transmitters that send a signal to an instrument calibrated to read out the
height or volume.
In this module, we will examine the more challenging situations that require
inferential level measurement. This technique obtains a level indication
indirectly by monitoring the pressure exerted by the height of the liquid in
the vessel.
The pressure at the base of a vessel containing liquid is directly proportional
to the height of the liquid in the vessel. This is termed hydrostatic pressure.
As the level in the vessel rises, the pressure exerted by the liquid at the base
of the vessel will increase linearly. Mathematically, we have:
P = S⋅H
where
P = Pressure (Pa)
3
S = Weight density of the liquid (N/m ) = ρg
H = Height of liquid column (m)
ρ = Density (kg/m3)
2
g = acceleration due to gravity (9.81 m/s )
The level of liquid inside a tank can be determined from the pressure
reading if the weight density of the liquid is constant.
Differential Pressure (DP) capsules are the most commonly used devices to
measure the pressure at the base of a tank.
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When a DP transmitter is used for the purpose of measuring a level, it will
be called a level transmitter.
To obtain maximum sensitivity, a pressure capsule has to be used, that has a
sensitivity range that closely matches the anticipated pressure of the
measured liquid. However, system pressures are often much higher than the
actual hydrostatic pressure that is to be measured. If the process pressure is
accidentally applied to only one side of the DP capsule during installation or
removal of the DP cell from service, over ranging of the capsule would
occur and the capsule could be damaged causing erroneous indications.
2.3.2 Three Valve Manifold
A three-valve manifold is a device that is used to ensure that the capsule will
not be over-ranged. It also allows isolation of the transmitter from the
process loop. It consists of two block valves - high pressure and lowpressure block valve - and an equalizing valve. Figure 1 shows a three valve
manifold arrangement.
ProcessHigh Pressure Side
ProcessLow Pressure Side
HP
Block
Valve
LP
Block
Valve
3 Valve
Manifold
Equalizing Valve
HP
LT
LP
Signal
Figure 1
A Three Valve Manifold
During normal operation, the equalizing valve is closed and the two block
valves are open. When the transmitter is put into or removed from service,
the valves must be operated in such a manner that very high pressure is
never applied to only one side of the DP capsule.
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Operational Sequences of Three-Valve Manifold Valving Transmitter
into Service
To valve a DP transmitter into service an operator would perform the
following steps:
1.
Check all valves closed.
2.
Open the equalizing valve – this ensures that the same
pressure will be applied to both sides of the transmitter, i.e.,
zero differential pressure.
3.
Open the High Pressure block valve slowly, check for
leakage from both the high pressure and low-pressure side of
the transmitter.
4.
Close the equalizing valve – this locks the pressure on both
sides of the transmitter.
5.
Open the low-pressure block valve to apply process pressure
to the low-pressure side of the transmitter and establish the
working differential pressure.
6.
The transmitter is now in service.
Note it may be necessary to bleed any trapped air from the capsule housing.
Removing Transmitter from Service
Reversal of the above steps allows the DP transmitter to be removed from
service.
1.
Close the low-pressure block valve.
2.
Open the equalizing valve.
3.
Close the high-pressure block valve.
The transmitter is now out of service.
Note the transmitter capsule housing still contains process pressure; this will
require bleeding.
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2.3.3 Open Tank Measurement
The simplest application is the fluid level in an open tank. Figure 2 shows a
typical open tank level measurement installation using a pressure capsule
level transmitter.
Atmospheric Pressure P
H
atm
Vented
to
Atmosphere
Liquid of Weight Density S
HP
Isolating
Valve
LT
LP
Figure 2
Open Tank Level Measurement Installation
If the tank is open to atmosphere, the high-pressure side of the level
transmitter will be connected to the base of the tank while the low-pressure
side will be vented to atmosphere. In this manner, the level transmitter acts
as a simple pressure transmitter. We have:
Phigh = Patm + S ⋅ H
Plow = Patm
Differential pressure ∆P = Phigh - Plow = S ⋅ H
The level transmitter can be calibrated to output 4 mA when the tank is at
0% level and 20 mA when the tank is at 100% level.
2.3.4 Closed Tank Measurement
Should the tank be closed and a gas or vapour exists on top of the liquid, the
gas pressure must be compensated for. A change in the gas pressure will
cause a change in transmitter output. Moreover, the pressure exerted by the
gas phase may be so high that the hydrostatic pressure of the liquid column
becomes insignificant. For example, the measured hydrostatic head in a
CANDU boiler may be only three meters (30 kPa) or so, whereas the steam
pressure is typically 5 MPa. Compensation can be achieved by applying the
gas pressure to both the high and low-pressure sides of the level transmitter.
This cover gas pressure is thus used as a back pressure or reference pressure
on the LP side of the DP cell. One can also immediately see the need for the
three-valve manifold to protect the DP cell against these pressures.
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The different arrangement of the sensing lines to the DP cell is indicated a
typical closed tank application (figure 3).
Figure 3 shows a typical closed tank installation.
Isolation
Valve
P gas
Low Pressure
Impulse Line
Isolation
Valve
LT
High
Low
4-20mA
Signal
Figure 3
Typical Closed Tank Level Measurement System
We have:
Phigh = Pgas + S ⋅ H
Plow = Pgas
∆P = Phigh - Plow = S ⋅ H
The effect of the gas pressure is cancelled and only the pressure due to the
hydrostatic head of the liquid is sensed. When the low-pressure impulse line
is connected directly to the gas phase above the liquid level, it is called a dry
leg.
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Dry Leg System
A full dry leg installation with three-valve manifold is shown in Figure 4
below.
Isolation
Valve
P gas
Low Pressure
Impulse Line
Isolation
Valve
3 Valve
Manifold
Low
High
LT
Isolating Valve
(normally open)
Knock-out Pot
Drain Valve
(normally closed)
Figure 4
Dry Leg Installation with Three-Valve Manifold
If the gas phase is condensable, say steam, condensate will form in the lowpressure impulse line resulting in a column of liquid, which exerts extra
pressure on the low-pressure side of the transmitter. A technique to solve
this problem is to add a knockout pot below the transmitter in the lowpressure side as shown in Figure 4. Periodic draining of the condensate in
the knockout pot will ensure that the impulse line is free of liquid.
In practice, a dry leg is seldom used because frequent maintenance is
required. One example of a dry leg application is the measurement of liquid
poison level in the poison injection tank, where the gas phase is noncondensable helium. In most closed tank applications, a wet leg level
measurement system is used.
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Wet Leg System
In a wet leg system, the low-pressure impulse line is completely filled with
liquid (usually the same liquid as the process) and hence the name wet leg.
A level transmitter, with the associated three-valve manifold, is used in an
identical manner to the dry leg system.
Figure 5 shows a typical wet leg installation.
Pressure Release
Valve
Isolating Valve 1
P gas
Sloped towards
main tank
Steam or Electric
Heating
Isolating
Valve 2
Drain
Valves
3 Valve
Manifold
High
LT
Low
Transmitter Drain Valves
Figure 5
A Wet Leg Installation
At the top of the low pressure impulse line is a small catch tank. The gas
phase or vapour will condense in the wet leg and the catch tank. The catch
tank, with the inclined interconnecting line, maintains a constant hydrostatic
pressure on the low-pressure side of the level transmitter. This pressure,
being a constant, can easily be compensated for by calibration. (Note that
operating the three-valve manifold in the prescribed manner helps to
preserve the wet leg.)
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If the tank is located outdoors, trace heating of the wet leg might be
necessary to prevent it from freezing. Steam lines or an electric heating
element can be wound around the wet leg to keep the temperature of the
condensate above its freezing point.
Note the two sets of drain valves. The transmitter drain valves would be
used to drain (bleed) the transmitter only. The two drain valves located
immediately above the three-valve manifold are used for impulse and wet
leg draining and filling.
In addition to the three-valve manifold most transmitter installations have
valves where the impulse lines connect to the process. These isolating
valves, sometimes referred to as the root valves, are used to isolate the
transmitter for maintenance.
Level Compensation
It would be idealistic to say that the DP cell can always be located at the
exact the bottom of the vessel we are measuring fluid level in. Hence, the
measuring system has to consider the hydrostatic pressure of the fluid in the
sensing lines themselves. This leads to two compensations required.
Zero Suppression
In some cases, it is not possible to mount the level transmitter right at the
base level of the tank. Say for maintenance purposes, the level transmitter
has to be mounted X meters below the base of an open tank as shown in
Figure 6.
Isolating
Valve
H
H.P. Impulse Line
Vented to Atmosphere
Xm
HP
LT
LP
Figure 6
Level Transmitter with Zero Suppression
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The liquid in the tank exerts a varying pressure that is proportional to its
level H on the high-pressure side of the transmitter. The liquid in the highpressure impulse line also exerts a pressure on the high-pressure side.
However, this pressure is a constant (P = S ⋅ X ) and is present at all times.
When the liquid level is at H meters, pressure on the high-pressure side of
the transmitter will be:
Phigh = S ⋅ H + S ⋅ X + Patm
Plow = Patm
∆P
= Phigh - Plow = S ⋅ H + S ⋅ X
That is, the pressure on the high-pressure side is always higher than the
actual pressure exerted by the liquid column in the tank (by a value of
S ⋅ X ). This constant pressure would cause an output signal that is higher
than 4 mA when the tank is empty and above 20 mA when it is full. The
transmitter has to be negatively biased by a value of - S ⋅ X so that the output
of the transmitter is proportional to the tank level ( S ⋅ H ) only. This
procedure is called Zero Suppression and it can be done during calibration
of the transmitter. A zero suppression kit can be installed in the transmitter
for this purpose.
Zero Elevation
When a wet leg installation is used (see Figure 7 below), the low-pressure
side of the level transmitter will always experience a higher pressure than
the high-pressure side. This is due to the fact that the height of the wet leg
(X) is always equal to or greater than the maximum height of the liquid
column (H) inside the tank.
When the liquid level is at H meters, we have:
Phigh = Pgas + S ⋅ H
Plow = Pgas + S ⋅ X
∆P = Phigh - Plow = S ⋅ H - S ⋅ X
= - S (X - H)
The differential pressure ∆P sensed by the transmitter is always a negative
number (i.e., low pressure side is at a higher pressure than high pressure
side). ∆P increases from P = - S ⋅ X to P = -S (X-H) as the tank level rises
from 0% to 100%.
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If the transmitter were not calibrated for this constant negative error (S ⋅ X ), the transmitter output would read low at all times.
To properly calibrate the transmitter, a positive bias (+ S ⋅ X ) is needed to
elevate the transmitter output.
This positive biasing technique is called zero elevation.
Steam Outlet
Condensate Pot
Steam
(Pgas)
L.P. Impulse
Line filled with
H2O
Xm
Hm
Hot Water
H
L
Water Inlet
Figure 7
Requirement for Zero Elevation
2.3.5
Bubbler Level Measurement System
If the process liquid contains suspended solids or is chemically corrosive or
radioactive, it is desirable to prevent it from coming into direct contact with
the level transmitter. In these cases, a bubbler level measurement system,
which utilizes a purge gas, can be used.
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Open Tank Application for Bubbler System
Figure 8 illustrates a typical bubbler system installation.
Note
4-20mA
Signal
Constant Differential
Pressure Relay
H L
Purge Gas
Supply
Vented to
Atmosphere
Bubbler Tube
H
Reference
Figure 8
Bubbler Level Measurement System in Open Tank Application
As shown in Figure 8, a bubbler tube is immersed to the bottom of the
vessel in which the liquid level is to be measured. A gas (called purge gas)
is allowed to pass through the bubbler tube. Consider that the tank is empty.
In this case, the gas will escape freely at the end of the tube and therefore
the gas pressure inside the bubbler tube (called back pressure) will be at
atmospheric pressure. However, as the liquid level inside the tank increases,
pressure exerted by the liquid at the base of the tank (and at the opening of
the bubbler tube) increases. The hydrostatic pressure of the liquid in effect
acts as a seal, which restricts the escape of, purge gas from the bubbler tube.
As a result, the gas pressure in the bubbler tube will continue to increase
until it just balances the hydrostatic pressure (P = S ⋅ H ) of the liquid. At
this point the backpressure in the bubbler tube is exactly the same as the
hydrostatic pressure of the liquid and it will remain constant until any
change in the liquid level occurs. Any excess supply pressure will escape as
bubbles through the liquid.
As the liquid level rises, the backpressure in the bubbler tube increases
proportionally, since the density of the liquid is constant.
A level transmitter (DP cell) can be used to monitor this backpressure. In an
open tank installation, the bubbler tube is connected to the high-pressure
side of the transmitter, while the low pressure side is vented to atmosphere.
The output of the transmitter will be proportional to the tank level.
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A constant differential pressure relay is often used in the purge gas line to
ensure that constant bubbling action occurs at all tank levels. The constant
differential pressure relay maintains a constant flow rate of purge gas in the
bubbler tube regardless of tank level variations or supply fluctuation. This
ensures that bubbling will occur to maximum tank level and the flow rate
does not increase at low tank level in such a way as to cause excessive
disturbances at the surface of the liquid. Note that bubbling action has to be
continuous or the measurement signal will not be accurate.
An additional advantage of the bubbler system is that, since it measures only
the backpressure of the purge gas, the exact location of the level transmitter
is not important. The transmitter can be mounted some distance from the
process. Open loop bubblers are used to measure levels in spent fuel bays.
Closed Tank Application for Bubbler System
If the bubbler system is to be applied to measure level in a closed tank, some
pressure-regulating scheme must be provided for the gas space in the tank.
Otherwise, the gas bubbling through the liquid will pressurize the gas space
to a point where bubbler supply pressure cannot overcome the static
pressure it acts against. The result would be no bubble flow and, therefore,
inaccurate measurement signal. Also, as in the case of a closed tank
inferential level measurement system, the low-pressure side of the level
transmitter has to be connected to the gas space in order to compensate for
the effect of gas pressure.
Some typical examples of closed tank application of bubbler systems are the
measurement of water level in the irradiated fuel bays and the light water
level in the liquid zone control tanks.
2.3.6 Effect of Temperature on Level Measurement
Level measurement systems that use differential pressure ∆P as the sensing
method, are by their very nature affected by temperature and pressure.
Recall that the measured height H of a column of liquid is directly
proportional to the pressure P exerted at the base of the column and
inversely proportional to the density ρ of the liquid.
H α P/ρ
Density (mass per unit volume) of a liquid or gas is inversely proportional to
its temperature.
ρ α 1/T
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Thus, for any given amount of liquid in a container, the pressure P exerted at
the base will remain constant, but the height will vary directly with the
temperature.
HαT
Consider the following scenario. A given amount of liquid in a container
[figure 9(a)] is exposed to higher process temperatures [figure 9(b)].
Atmospheric Pressure P
H1
atm
Vented
to
Atmosphere
Liquid of Density ρ 1
Temperature T 1
HP
Isolating
Valve
LT
LP
Figure 9(a)
Low Process Temperature
Atmospheric Pressure Patm
H2
Vented
to
Atmosphere
Liquid of Density ρ2
Temperature T2
HP
Isolating
Valve
LT
LP
Figure 9(b)
High Process Temperature
As the amount (mass) of liquid does not change from figure 9(a) to 9(b), the
pressure exerted on the base of the container has not changed and the
indicated height of the liquid does not change. However, the volume
occupied by the liquid has increased and thus the actual height has
increased.
The above scenario of figure (9) is a common occurrence in plant
operations. Consider a level transmitter calibrated to read correctly at 750C.
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If the process temperature is increased to 900C as in figure 9 (c), the actual
level will be higher than indicated.
The temperature error can also occur in wet-leg systems (figure 10).
Isolating
Valve
P gas
Process
Temperature T
H1
LP
1
Liquid of Density ρ 1
HP
Isolating
Valve
LT
Figure 10
Temperature Effect on Wet-Leg System
If the reference leg and variable leg are at the same temperature that the
level transmitter (LT) is calibrated for, the system will accurately measure
liquid level. However, as the process temperature increases, the actual
process fluid level increases (as previously discussed), while the indicated
measurement remains unchanged.
Further errors can occur if the reference leg and the variable (sensing) leg
are at different temperatures. The level indication will have increasing
positive (high) error as the temperature of the wet reference leg increases
above the variable (process) leg.
As an example, consider temperature changes around a liquid storage tank
with a wet leg. As temperature falls and the wet leg cools off, the density of
the liquid inside it increases, while the temperature in the tank remains
practically unchanged (because of a much bigger volume and connection to
the process). As a result the pressure of the reference leg rises and the
indicated level decreases. If it happens to the boiler level measurement for a
shutdown system it can even lead to an unnecessary reactor trip on boiler
low level. However, high-level trips may be prevented under these
circumstances. In an extreme case the wet leg may freeze invalidating the
measurement scheme completely, but it could be easily prevented with trace
heating as indicated earlier (Figure 5).
False high level indication can be caused by an increased wet leg
temperature, gas or vapour bubbles or a drained wet leg.
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A high measured tank level, with the real level being dangerously low, may
prevent the actuation of a safety system on a low value of the trip parameter.
The real level may even get sufficiently low to cause either the cavitation of
the pumps that take suction from the tank or gas ingress into the pumps and
result in gas locking and a reduced or no flow condition. If the pumps are
associated with a safety system like ECI or a safety related system like PHT
shutdown cooling, it can lead to possible safety system impairments and
increased probability of resultant fuel damage.
2.3.7 Effect of Pressure on Level Measurement
Level measurement systems that use differential pressure ∆P as the sensing
method, are also affected by pressure, although not to the same degree as
temperature mentioned in the previous section.
Again the measured height H of a column of liquid is directly proportional
to the pressure PL exerted at the base of the column by the liquid and
inversely proportional to the density ρ of the liquid:
H α PL/ρ
Density (mass per unit volume) of a liquid or gas is directly proportional to
the process or system pressure Ps.
ρ α Ps
Thus, for any given amount of liquid in a container, the pressure PL (liquid
pressure) exerted at the base of the container by the liquid will remain
constant, but the height will vary inversely with the process or system
pressure.
H α 1/Ps
Most liquids are fairly incompressible and the process pressure will not
affect the level unless there is significant vapour content.
2.3.8 Level Measurement System Errors
The level measurement techniques described in this module use inferred
processes and not direct measurements. Namely, the indication of fluid level
is based on the pressure exerted on a differential pressure (DP) cell by the
height of the liquid in the vessel. This places great importance on the
physical and environmental problems that can affect the accuracy of this
indirect measurement.
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Connections
As amusing as it may sound, many avoidable errors occur because the DP
cell had the sensing line connections reversed.
In systems that have high operating pressure but low hydrostatic pressure
due to weight of the fluid, this is easy to occur. This is particularly important
for closed tank systems.
With an incorrectly connected DP cell the indicated level would go down
while the true tank level increases.
Over-Pressuring
Three valve manifolds are provided on DP cells to prevent over-pressuring
and aid in the removal of cells for maintenance. Incorrect procedures can
inadvertently over-pressure the differential pressure cell. If the cell does not
fail immediately the internal diaphragm may become distorted. The
measurements could read either high or low depending on the mode of
failure.
Note that if the equalizing valve on the three-valve manifold is inadvertently
opened, the level indication will of course drop to a very low level as the
pressure across the DP cell equalizes.
Sensing lines
The sensing lines are the umbilical cord to the DP cell and must be
functioning correctly. Some of the errors that can occur are:
Obstructed sensing lines
The small diameter lines can become clogged with particulate, with
resulting inaccurate readings. Sometimes the problem is first noted as an
unusually sluggish response to a predicted change in level. Periodic draining
and flushing of sensing lines is a must.
Draining sensing lines
As mentioned previously, the lines must be drained to remove any debris or
particulate that may settle to the bottom of the tank and in the line. Also, in
closed tank dry leg systems, condensate must be removed regularly to
prevent fluid pressure building up on the low-pressure impulse line. Failure
to do so will of course give a low tank level reading. Procedural care must
be exercised to ensure the DP cell is not over-ranged inadvertently during
draining. Such could happen if the block valves are not closed and
equalizing valve opened beforehand.
False high level indication can be caused by a leaking or drained wet leg.
A leaking variable (process) leg can cause false low-level indication.
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2.4
TEMPERATURE MEASUREMENT
Every aspect of our lives, both at home and at work, is influenced by
temperature. Temperature measuring devices have been in existence for
centuries. The age-old mercury in glass thermometer is still used today and
why not? The principle of operation is ageless as the device itself. Its
operation was based on the temperature expansion of fluids (mercury or
alcohol). As the temperature increased the fluid in a small reservoir or bulb
expanded and a small column of the fluid was forced up a tube. You will
find the same theory is used in many modern thermostats today. In this
module we will look at the theory and operation of some temperature
measuring devices commonly found in a generating station. These include
thermocouples, thermostats and resistive temperature devices.
Thermocouples (T/C) and resistive temperature devices (RTD) are generally
connected to control logic or instrumentation for continuous monitoring of
temperature. Thermostats are used for direct positive control of the
temperature of a system within preset limits.
2.4.1
49
Resistance Temperature Detector (RTD)
Every type of metal has a unique composition and has a different resistance
to the flow of electrical current. This is termed the resistively constant for
that metal. For most metals the change in electrical resistance is directly
proportional to its change in temperature and is linear over a range of
temperatures. This constant factor called the temperature coefficient of
electrical resistance (short formed TCR) is the basis of resistance
temperature detectors. The RTD can actually be regarded as a high
precision wire wound resistor whose resistance varies with temperature. By
measuring the resistance of the metal, its temperature can be determined.
Several different pure metals (such as platinum, nickel and copper) can be
used in the manufacture of an RTD. A typical RTD probe contains a coil of
very fine metal wire, allowing for a large resistance change without a great
space requirement. Usually, platinum RTDs are used as process
temperature monitors because of their accuracy and linearity.
To detect the small variations of resistance of the RTD, a temperature
transmitter in the form of a Wheatstone bridge is generally used. The circuit
compares the RTD value with three known and highly accurate resistors.
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R3
Connecting
Wires
Note
R1
A
B
Power
Supply
R2
RTD
Field
Control Room
Figure 1
RTD using a Wheatstone Bridge
A Wheatstone bridge consisting of an RTD, three resistors, a voltmeter and
a voltage source is illustrated in Figure 1. In this circuit, when the current
flow in the meter is zero (the voltage at point A equals the voltage at point
B) the bridge is said to be in null balance. This would be the zero or set
point on the RTD temperature output. As the RTD temperature increases,
the voltage read by the voltmeter increases. If a voltage transducer replaces
the voltmeter, a 4-20 mA signal, which is proportional to the temperature
range being monitored, can be generated.
As in the case of a thermocouple, a problem arises when the RTD is
installed some distance away from the transmitter. Since the connecting
wires are long, resistance of the wires changes as ambient temperature
fluctuates. The variations in wire resistance would introduce an error in the
transmitter. To eliminate this problem, a three-wire RTD is used.
R3
R1
A
B
RW1
4-20mA
Detector
3- Wire RTD
RW2
RW3
Field
Control Room
Figure 2
Three-Wired RTD
Figure 2 illustrates a three-wire RTD installation.
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Power
Supply
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The connecting wires (w1, w2, w3) are made the same length and therefore
the same resistance. The power supply is connected to one end of the RTD
and the top of the Wheatstone bridge. It can be seen that the resistance of
the right leg of the Wheatstone bridge is R1 + R2 + RW2. The resistance of
the left leg of the bridge is R3 + RW3 + RTD. Since RW1 = RW2, the result is
that the resistances of the wires cancel and therefore the effect of the
connecting wires is eliminated.
RTD Advantages and Disadvantages
Advantages:
•
The response time compared to thermocouples is very fast – in the
order of fractions of a second.
•
An RTD will not experience drift problems because it is not selfpowered.
•
Within its range it is more accurate and has higher sensitivity than a
thermocouple.
•
In an installation where long leads are required, the RTD does not
require special extension cable.
•
Unlike thermocouples, radioactive radiation (beta, gamma and
neutrons) has minimal effect on RTDs since the parameter measured
is resistance, not voltage.
Disadvantages:
•
Because the metal used for a RTD must be in its purest form, they
are much more expensive than thermocouples.
•
In general, an RTD is not capable of measuring as wide a
temperature range as a thermocouple.
•
A power supply failure can cause erroneous readings
•
Small changes in resistance are being measured, thus all connections
must be tight and free of corrosion, which will create errors.
•
Among the many uses in a nuclear station, RTDs can be found in the
reactor area temperature measurement and fuel channel coolant
temperature.
Failure Modes:
•
An open circuit in the RTD or in the wiring between the RTD and
the bridge will cause a high temperature reading.
•
Loss of power or a short within the RTD will cause a low
temperature reading.
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2.4.2
52
Thermocouple (T/C)
A thermocouple consists of two pieces of dissimilar metals with their ends
joined together (by twisting, soldering or welding). When heat is applied to
the junction, a voltage, in the range of milli-volts (mV), is generated. A
thermocouple is therefore said to be self-powered. Shown in Figure 3 is a
completed thermocouple circuit.
Junction 1 at
Temperature T1
Metal B
Metal A
Junction 2 at
Temperature T2
Metal A
Galvanometer
Galvanometer
Figure 3
A Thermocouple Circuit
The voltage generated at each junction depends on junction temperature. If
temperature T1 is higher than T2, then the voltage generated at Junction 1
will be higher than that at Junction 2. In the above circuit, the loop current
shown on the galvanometer depends on the relative magnitude of the
voltages at the two junctions.
In order to use a thermocouple to measure process temperature, one end of
the thermocouple has to be kept in contact with the process while the other
end has to be kept at a constant temperature. The end that is in contact with
the process is called the hot or measurement junction. The one that is kept
at constant temperature is called cold or reference junction. The relationship
between total circuit voltage (emf) and the emf at the junctions is:
Circuit emf = Measurement emf - Reference emf
If circuit emf and reference emf are known, measurement emf can be
calculated and the relative temperature determined.
To convert the emf generated by a thermocouple to the standard 4-20 mA
signal, a transmitter is needed. This kind of transmitter is called a
temperature transmitter. Figure 4 shows a simplified temperature
transmitter connection.
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Temperature
Transmitter
Note
Metal A
Measurement
Junction at
Process
Temperature
4-20mA
Reference
Junction
Metal B
Metal C
Field
Control Room
Figure 4
A Simplified Thermocouple Temperature Transmitter
In Figure 4 above, the temperature measurement circuit consists of a
thermocouple connected directly to the temperature transmitter. The hot and
cold junctions can be located wherever required to measure the temperature
difference between the two junctions.
In most situations, we need monitor the temperature rise of equipment to
ensure the safe operation. Temperature rise of a device is the operating
temperature using ambient or room temperature as a reference. To
accomplish this the hot junction is located in or on the device and the cold
junction at the meter or transmitter as illustrated in figure 5.
Temperature
Transmitter
Metal C
Metal A
Measurement
Junction
Metal B
Field
Control Room
Reference
Junction
Figure 5
Typical Thermocouple Circuit
Thermocouple Advantages and Disadvantages
Advantages:
•
Thermocouples are used on most transformers. The hot junction is
inside the transformer oil and the cold junction at the meter mounted
on the outside. With this simple and rugged installation, the meter
directly reads the temperature rise of oil above the ambient
temperature of the location.
•
In general, thermocouples are used exclusively around the turbine
hall because of their rugged construction and low cost.
•
A thermocouple is capable of measuring a wider temperature range
than an RTD.
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Disadvantages:
•
If the thermocouple is located some distance away from the
measuring device, expensive extension grade thermocouple wires or
compensating cables have to be used.
•
Thermocouples are not used in areas where high radiation fields are
present (for example, in the reactor vault). Radioactive radiation
(e.g., Beta radiation from neutron activation), will induce a voltage
in the thermocouple wires. Since the signal from thermocouple is
also a voltage, the induced voltage will cause an error in the
temperature transmitter output.
•
Thermocouples are slower in response than RTDs
•
If the control logic is remotely located and temperature transmitters
(milli-volt to milli- amp transducers) are used, a power supply
failure will of course cause faulty readings.
Failure Modes:
An open circuit in the thermocouple detector means that there is no path for
current flow, thus it will cause a low (off-scale) temperature reading.
A short circuit in the thermocouple detector will also cause a low
temperature reading because it creates a leakage current path to the ground
and a smaller measured voltage.
2.4.3
Thermal Wells
The process environment where temperature monitoring is required, is often
not only hot, but also pressurized and possibly chemically corrosive or
radioactive. To facilitate removal of the temperature sensors (RTD and TC),
for examination or replacement and to provide mechanical protection, the
sensors are usually mounted inside thermal wells (Figure 6).
Process Connecting Threads
Protective Well
This portion in process
Connector Block
Figure 6
Typical Thermal Well Installation
A thermal well is basically a hollow metal tube with one end sealed. It is
usually mounted permanently in the pipe work. The sensor is inserted into it
and makes contact with the sealed end.
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Re
spo
nse
Ba
re
Temperature
A drawback to thermal wells is their long response time because heat must
be transferred through the well to the sensor. An example of the
temperature response for bare and thermal well installed sensors is shown in
Figure 7. Minimizing the air space between the sensor and the well,
however, can decrease this thermal lag.
Sensor in Well Response
Time
Figure 7
Response Curves of Bare and Thermal Well Installation
2.4.4 Thermostats
Thermostats have a different function than the resistive temperature
detectors and thermocouples that we have just discussed. The thermostats
directly regulate the temperature of a system by maintaining it constant or
varying it over a specific range. The T/C or RTD could be used as the
temperature-sensing element of a thermostat, but generally thermostats are
direct acting devices.
The two common types of thermostats are:
Pressure cylinder
Bimetallic strip
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Pressure Cylinders
The most common thermostat depends on the expansion of a fluid such as
mercury or a solid with an increase in temperature as in figure 8.
A
B
Figure 8
Thermostat Pressure Cylinder
The plunger connected to the piston is used to force contacts open and
closed to control valve positions or pump control. Often the plunger is
directly connected to the valve as in figure 9 below. This is the same
principle as used in automobile water thermostats where the substance in the
cylinder is a wax with a melting point of around 1800 F.
A
B
Figure 9
Thermostat Pressure Cylinder Application
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Bimetallic Strips
A bimetallic strip is constructed by bonding two metals with different
coefficients of thermal expansion (Figure 10). If heat is applied to one end
of the strip, the metal with the higher coefficient of expansion will expand
more readily than the lower one. As a result, the whole metallic strip will
bend in the direction of the metal with the lower coefficient (Figure 11).
Low C. of E.
Fixe
Fixed Point
Ambient
Temp.
High C. of E.
Figure 10
A Bimetallic Strip
Direction of
Movement
Fixed Point
bient
p.
Figure 11
Bimetallic Strip Bent after Heat is Applied
When contacts are attached to the strip, it can be used as a fast acting
thermostat to control air temperature as per figure 12. One drawback is that
there cannot be any flammable vapours surrounding the strip due to arcing
generated across the contacts.
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Note
Figure 12
Bimetallic Thermostat
One main advantage of the bimetallic strip is that it can be used to operate
over a range of temperatures when the strip is fashioned into a coil (for
larger swing) and placed on an adjustable pivot (figure 13). Most room
thermostats operate on this principle.
Figure 13
Application of Bimetallic Strip
Bimetallic Thermometers
Another common configuration of the bimetallic strip is coiled in a helix to
increase the swing or displacement similar to the coil above. In this shape,
the strip is more rugged and less subject to vibration. A helical bimetallic
thermometer is shown in Figure 14 below.
Bimetallic thermometers in general are very rugged and require little
maintenance. They are usually used to measure process parameters such as
pump and bearing temperature.
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Pointer
Note
Instrument Case
Helical Bimetal
Figure 14
Helical Bimetallic Thermometer
2.5 NEUTRON FLUX MEASUREMENT
A CANDU reactor generates thermal power from heat produced by nuclear
fission. However, measuring the actual thermal power output proves to be
too slow for the purpose of controlling the reactor (there is a time lag of
about 25 seconds between neutron flux change and thermal output
measurement). To control the reactor adequately, it is necessary to have fast
responding instrumentation. The method of measuring reactor power by
observing the radiation directly associated with the fission process is used.
Each time a fission occurs; radiation (neutron, beta, gamma or alpha) is
produced. The magnitude of these radiations is directly proportional to the
number of fissions, which is in turn directly related to the reactor power
level. The radiation we choose to monitor is neutron flux. If we can measure
the neutron flux, we can estimate the thermal output of the reactor. In
addition, monitoring neutron flux acts as a safeguard against the possibility
of losing control of the reactor. If neutron flux exceeds a predetermined
limit, shutdown systems will be triggered by the sensed neutron signal.
2.5.1 Neutron Flux Detection
Power produced by thermal nuclear reactors is generated through fission
induced by slow neutrons. Hence, nuclear sensors that are part of the reactor
control or safety systems are generally based on detectors that respond
primarily to slow neutrons. The control of a reactor is necessary over a wide
range of neutron flux levels from zero to full reactor power at 1014 n/(cm2 s).
The level of 107 n/(cm2s) is approximately zero thermal power for all
practical purposes and significant thermal power contribution does not occur
until the flux rises to ~1012 (1% FP). The normal operating range of the
reactor is in usually between 1013 and 1014 n/(cm2 s).
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It is more convenient to describe the flux density in terms of the logarithmic
value, with 14 being full power (1014 N/(cm2 s) = 100%FP). By
terminology, the neutron flux is said to have gone through 14 decades from
zero to full power.
In order to provide instrumentation that will have complete coverage over
this large range and still maintain accuracy, it is necessary to split the
detection into three areas of sensitivity.
1. Sub-critical α 0 to 107 n/(cm2 s). Neutron detection in this region
is covered by two sets of start-up instrumentation:
•
In core BF3 proportional neutron counters with a
range 10-14 to 10-10 FP.
•
Out of core He-3 proportional neutron counters with a
range of 10-11 to 10-6 FP.
2. Run up α 107 to 1013 n/(cm2 s). Primary detection by external to
core ion chambers. With a range of 10-7 to 1.5 FP.
3. Under-load α 1013 to 1014 n/(cm2 s). Primary detection by
internal core neutron detectors with a range of 0.15 to 1.5 FP.
As there are different detectors used, overlap of the detectors is necessary to
provide a smooth transfer of reactor control throughout the three areas and
to provide backup instrumentation if the main detectors are in question. We
will look at each of these areas in more detail.
2.5.2 Neutron Detection Methods
Neutrons, like gamma rays, have no charge and therefore cannot directly
interact with instrumentation, as do charged particles and electrons.
Neutrons can also travel through many centimeters of matter without any
type of interaction and thus can be totally invisible to a detector of common
size. When a neutron does interact it is with a nucleus of the absorbing
material. This interaction may either cause the neutron to disappear totally
and be replaced by one or more secondary radiations or change the energy
or direction of the neutron significantly.
Secondary radiations resulting from neutron interactions are almost always
heavy charged particles. These particles may be produced either as a result
of neutron-induced nuclear reactions or they may be the nuclei of the
absorbing material itself which have gained energy from the neutron
collisions.
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Most neutron detectors utilize some means of converting the incident
neutron into secondary charged particles that can then be detected directly.
For slow (thermal) neutrons, which have a small amount of kinetic energy,
several different neutron reactions can be used. These include; (l) prompt
capture of neutrons resulting in charged particle emission, (2) delayed
activation reactions where an activated nucleus emits some form of radiation
within a convenient half-life and energy and (3) fission reaction resulting
from neutron capture.
2.5.3 Start-up (sub-critical) Instrumentation
If the reactor had been operating for some time and the shut-down is short
(max. 2-3 weeks), the fission products will have built up to a point where
the photo-neutron presence will still be large enough in the reactor to be
detected by the run-up instrumentation (ion chambers).
For longer shutdowns, where the power has dropped to a relatively low
level, the readings from the ion chambers are unreliable because the
background gamma radiation levels heavily influence them. After some
time, power will drop to a level where the installed instruments go off-scale
(below about 10-7 FP) and the regulating system will not automatically
control the power. Figure 1 shows a typical power decay curve for a
CANDU reactor. It can be seen that after about 3 weeks supplementary
proportional counters are necessary to maintain a reliable power record.
Moreover, a subsequent approach to critical would need to be done using the
proportional counters in the initial stages.
1.00E-03
Fraction of FP
1.00E-04
1.00E-05
1.00E-06
1.00E-07
1.00E-08
0
5
10
15
20
25
30
35
Days
Figure 1
Power Decrease Following Shutdown
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The start-up instrumentation is comprised of two separate portable neutron
detection systems, one for in-core and the other for out-of-core. These
systems have two decades of overlap.
The out-of-core detectors consist of He-3 filled proportional neutron
counters that can be located in the ion chamber shutter holes. As we have
indicated this instrumentation measures power from 10-11 to 10-6 FP.
The BF3 (boron tri-fluoride) in-core detectors consist of boron in fluoridefilled proportional neutron counters that can be installed in the in-core
detector holes. This instrumentation measures the range from 10-14 to 10-10
FP and is used for start-up with fresh fuel. The energy released per reaction
is higher in B-10 than He-3, which enables the BF3; counters to better
discriminate against background gamma pulses.
The BF3 proportional counter (Figure 2) is a self-amplifying device that uses
the ions originally produced to make other ions in the same region. Inside
the proportional counter is an electrode of fine wire along the axis of the
second electrode that is a hollow cylinder. The effect of the wire is to give
strong electric field strengths close to it. This strong field quickly
accelerates the primary ions to gain enough energy in the acceleration to
produce secondary ionization pairs. These newly formed secondary ions are
also accelerated causing additional ionizations. The large number of events,
known as a Townsend avalanche, creates a single, large electrical pulse.
Since a single pulse is produced for each incident radiation particle or
photon, it is easy to directly measure the number of incident neutrons, which
interacted with the detector.
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n
Note
BF3 GAS
Figure 2
BF3 Proportional Counter
BF3 (boron tri-fluoride) gases used in proportional counters are enriched to
about 96% in B-10, which results in a level of efficiency five times greater
than the natural boron gas. BF3 tubes can readily discriminate against
gamma rays that are often found with the neutron flux being measured.
Gamma rays interact primarily in the wall of the counter and create
secondary electrons that may produce ionization in the gas. Because the
stopping power for electrons in BF3 is quite low, a typical electron will only
deposit a small fraction of its initial energy in the gas (little subsequent ionpair production) before reaching the opposite wall of the counter. Most
gamma ray interaction should result in low-amplitude pulses that can simply
be eliminated by an electronic amplitude discrimination circuit without
sacrificing detection efficiency.
These sensitive supplementary counters are removed once the standard ion
chambers take control. This prevents burnout of the instrumentation by
excessive neutron flux during normal reactor operation. The burnout is
caused by the rapid burn-up of the high cross section emitter material used
to obtain maximum sensitivity.
Due to the fast response time of the BF3 counters, the outputs are
deliberately damped at low flux levels to prevent spurious trips from the
shutdown systems.
2.5.4 Fission neutron detectors
The detectors that use the fission reaction to detect neutrons are called
fission chambers. These small ion chambers are typically made of stainless
steel walls and electrodes, with an operating voltage from 50V to 300V.
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The chamber walls are usually lined with highly enriched uranium to
enhance the ionization current.
Argon is the common choice for the chamber fill gas and it is used at a
pressure of several atmospheres. The elevated pressure ensures that the
range of fission fragments within the gas does not exceed the detector’s
small dimension. The pulses produced by fission fragments entering the
sensitive volume of the detector are large and because the ion chamber does
not produce large current flows, the output from the fission chamber is a
series of pulses that can be counted.
When the detector is operating, the fissionable material on the detector walls
is being consumed (by fission). To help slow the rate of depletion, a fast
neutron absorber such as 238U is sometimes added to the fissionable material
on the wall. When 238U absorbs a fast neutron, 239Pu is created after beta
emission. The 239Pu is itself a fissionable material.
Miniaturized fission chambers can be tailored for in-core use over any
power range likely to be encountered in reactor operation. These detectors
can be used as traveling detectors and as a reference point to calibrate selfpowered detectors.
2.5.5 Ion chamber neutron detectors
An ion chamber (see schematic in Figure 3) measures the electric charge of
ions generated from the interaction of neutrons and the chamber structure
and are located on the outside of the reactor core. The output of an ion
chamber is a flow of current directly proportional to the incident neutron
rate.
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under irradiation. It can either be nitrogen-helium or hydrogen. The housing
and central electrode is usually pure aluminum because of its low residual
activity as a result of neutron bombardment.
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Since neutrons are uncharged, the lining of the chamber must be a coating of
material, which will emit charged particles under neutron bombardment.
Boron-10 was chosen because its high cross-section for the (n, α) reaction
with thermal neutrons gives high sensitivity. This is important because ion
chambers are mounted outside the reactor core where the number of
neutrons is limited. Figure 4 indicates a representative location of the ion
chambers, although the positioning can vary.
SDS #1 Ion Chambers
RRS Ion Chambers
N
SDS #2 Ion Chambers
and spares
Figure 4
Typical Ion Chamber Locations
Ion Chamber Operation
The metal used for the electrode and outside casing is usually pure
aluminum because of its low residual activity as a result of neutron
bombardment. A high polarizing voltage (typically +600 V) is applied to the
aluminum housing, while the center electrode is normally kept at ground
potential (see Figure 3).
In the ion chamber, neutrons bombard the boron lining and release
positively charged alpha (α) particles. When a rapidly moving alpha particle
collides with a gas atom it ejects an electron from the atom leaving behind a
positively charged ion. This renders the gas conductive with so-called ion
pairs that are attracted to the charged electrodes by the polarizing voltage.
This creates a flow of electric current, which can be detected by an external
circuit. Amplifiers produce three separate signals as illustrated in Figure 5
then process the current signal:
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Note
Figure 5
Ion Chamber Neutron Detector System
•
A signal proportional to the linear power, lin n= 0 to 150%
FP
•
A signal proportional to the logarithm of the power, Log n
=-0 7 to 0 decades
•
A signal proportional to the rate of change of the logarithm of
power, log n rate = -15 to +15%/sec
At low reactor power level, say below 15% full power, control of bulk
reactor power is important. Ion chambers, because of their fast response
time and high sensitivity are used for low power neutron flux detection. Ion
chambers are used for the Reactor Regulating System (RRS) and Shut Down
Systems (SDS) because of their fast response time.
Gamma Discrimination for Neutron Ion Chambers
One of the problems with the ion chamber is that the detector is
indiscriminate and is affected by other ionizing radiation especially gamma.
(The external alpha and most beta radiation cannot penetrate the housing.)
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Gamma rays will produce high-energy electrons (termed photo-compton
electrons) and subsequent ionization. Therefore it is important to ensure
both at power and after shutdown, when fission gamma radiation is
predominant, that gamma radiation does not give a false (high) indication of
reactor power.
Discrimination against gamma rays is achieved by:
•
Employing appropriate materials in the detector and by
gamma shielding (lead) in the construction of the ion
chamber housings. With shielding the neutron to gamma
current ratio is kept at about 1000 to 1 at high power level.
•
Keeping the active part of the ion chamber relatively small.
Factors Affecting Ion Chamber Detector Accuracy
Proper functioning of ion chamber systems is essential to avoid unnecessary
shutdowns, especially at low power when the in-core flux detector system is
not capable of providing accurate data and therefore is not used.
A lower or negative signal can be caused by either the reactor power being
off scale (that is, less than 10-5% FP) or a failure in the system. The other
two channels will determine whether the power is off scale
A lower or off-scale reading can be caused by any of the following::
•
Low moderator level. If the moderator level drops, the
neutron velocity in the uncovered area will increase and will
be too fast to allow displacement of the alpha particles by the
Boron emitters of the ion chambers. The output of the ion
chamber will indicate a lower neutron flux reading.
•
Loss of HV power supply for the ion chamber polarization
voltage.
•
Shutdown of the Reactor. At less than 10-5% FP the output
will be zero or irrational. Normal nuclear instruments (ion
chambers and/or flux detectors) will be off-scale at their low
end (~10-5 % of full power) and so the regulating system will
not automatically control the reactor.
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Effect of Voltage on the Ionization Detector Process
Ionization detectors as previously discussed are a versatile instrument for
detection of radiation.
106
104
(2)
Continuous
Discharge Region
(1)
(3)
(4)
Geiger-Mueller
Region
108
Proportional Region
Relative Pulse Size
1010
Saturated Region
(Ion Chambers)
1012
Unsaturated Region
(Recombination)
1014
Limited Proportional
Region
These gas filled chambers can be operated as ion chambers, proportional
counters, or Geiger-Mueller (GM) tubes depending on the bias voltage
applied to the detector. Figure 7 illustrates the gas amplification curve (also
called the six-region curve) for gas filled detectors. It is a curve of the
relative electrical pulse size created at the collector compared to the voltage
between the emitter and collector.
(6)
(5)
Alpha
102
Beta
1
Gamma
Detector Bias Voltage
Figure 7
Six-Region Curve for Gas Filled Detectors
You will notice from the curve that at low voltages a high percentage of the
ions recombine before reaching the collector. This region is not usable for
detection purposes. As the bias voltage is increased into the ion chamber
region, a point is reached at which essentially all of the ions are collected
before they can recombine. No secondary ionization or gas amplification
occurs. At this point the output current will be proportional to the radiation
intensity and relatively independent of fluctuations in the power supply.
As the bias voltage is increased beyond the ion chamber region into the
proportional region, the ions created by primary ionization are accelerated
by the electric field towards the electrode. Unlike the ion chamber region,
however, the primary ions gain enough energy in the acceleration to produce
secondary ionization pairs. These newly formed secondary ions are also
accelerated, causing additional ionizations.
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The large number of events, known as an avalanche, creates a single
electrical pulse. In the proportional region, the detector output is
proportional to the total ionization product in the detector.
The ratio between the primary ionizations and the total number of ion pairs
produced is known as the gas amplification factor for that gas and can be in
the order of one million, compared to the gas amplification factor of one for
the ion chamber region.
As the bias voltage is increased beyond the proportional region, the detector
enters the limited proportional region and is unusable for detection purposes.
In this region, unpredictable avalanches can interfere with the overall signal.
As the bias voltage is increased even further into the Geiger-Mueller region,
the secondary ions are also accelerated to very high velocities and gain
sufficient energy to cause ionization themselves. The resulting avalanche
caused by a single ionization, results in a single very large pulse. The
avalanche continues until the large electrical field created by the ionization
interferes with the detector voltage field, decreasing the acceleration of the
ions and thus halting the avalanche.
At even higher voltages, the avalanche process cannot be halted, making this
region unsuitable for detection purposes.
2.5.6 In-Core Neutron Detectors
Although ion chambers are very accurate neutron detectors, their relatively
large size and delicate construction make them impractical to be used to
detect flux distribution inside the reactor. For this purpose, simple and
relatively inexpensive in-core detectors (ICD) have been developed. They
are also known as Hilborn detectors, self-powered neutron (SPN) detectors
or in-core flux detectors (ICFD).
The advantages of self-powered detectors when compared to other neutron
sensors include size, low cost and the relatively simple electronics required
to use them. Disadvantages stem from the low level of output current
produced by the devices, a relatively high sensitivity of the output current to
changes in the neutron energy spectrum and for many types, for instance
vanadium detectors, a rather slow response time.
The basic construction of an In-Core Detector (ICD) is shown in
Figure 7 with a platinum emitter, although other types of emitter materials
can be used.
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n
Platinum Emitter
Current
Meter
Inconel Sheath
Insulator
MgO Powder
Figure 7
Simplified In-Core Detector
The detector operates by generating an electric current after the central
emitter has captured the neutrons. This current is proportional to the rate at
which neutrons are captured in the detector. The detector itself resembles a
coaxial cable usually about a meter in length and consists of an outer inconel
(an alloy of nickel, chromium and iron) sheath and an inner emitter wire,
separated by a layer of insulation (usually magnesium oxide, MgO).
Magnesium oxide is most commonly used as it can withstand the extreme
temperature and radiation environment in the reactor core.
In-core detectors are usually denoted by the material of their emitters, which
in CANDU reactors are made of vanadium, platinum, platinum clad inconel
(inconel 600 core wire with a thin surface layer of platinum) and inconel.
Operation of these detectors requires no external power supply and that is
why they are called self-powered. The operation [see Figures 8(a) and 8(b)]
is quite simple. The current that corresponds to the beta rays given off by the
emitter is measured between the emitter and the outer shell, called the
collector.
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Note
Figure 8(a)
In-Core Detector Operation (a)
Figure 8(b)
In-Core Detector Operation (b)
Incident radiation (through the mechanisms described below) will cause
electrons to be emitted mostly from the emitter. The emitter is then
positively charged with respect to collector. The collected electrons pass
through the external amplifier and back to the emitter producing a useful
signal.
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The 3 principal components of the generated current and main interactions
are as follows:
•
(n, β) mechanism that is, beta emission [Figure 8(a)] following
neutron capture mostly by the emitter (neutron activation). This beta
decay current is proportional to the rate at which neutrons are
captured in the detector but has a rather slow response time.
•
(n, γ) followed by the emission of a Compton or photo electron, in
which electrons are produced by neutron capture gamma rays
[Figure 8(b)] mostly from the emitter. The radioactive capture
gamma rays are emitted within a very small fraction of a second
following neutron absorption.
•
Photo and Compton electrons are produced by gamma ray sources
external to the detector. These gammas will either be directly from
fission or from the decay of fission products (Figure 8).
External electrons and beta particles from the reactor hardware and
materials can also contribute to the detector signal, but this 4th mechanism
usually does not add more than a couple of per cent to the overall output
current.
In-core detectors can be either coiled (on an inner zircaloy tube) as shown in
Figure 9 or Straight Individually Replaceable (SIR) in a flux monitor
assembly. (ICD assemblies could be of various types, i.e., wet, encapsulated
and HESIR or hybrid encapsulated straight individually replaceable and
positioned either vertically or horizontally in the reactor core.)
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Control
Absorber
Locations
Detector
Locations
Top View of Reactor
Section
View
Outer Zircaloy
Tube
Core Boundary
Inner Zircaloy
Tube
Fuel Channels
C
C
Connecting
Leads
Two PlatinumEmitter Detectors
at each Location
C
C
C
These Detectors
(7 + 7) used for
control
S
C
57cm
C
Detectors at this location
used for safety (linear rate
of change of flux)
Figure 9
Typical ICD Locations
Types of In-Core Detectors
The time relationship between a device input and output is called its
dynamic response. Following a step change in reactor neutron flux the
current produced by each isotope in the ICD material through each of the
three basic interactions can be either a prompt (immediate) response or a
delayed one. Delayed responses are usually described by exponential lag
terms with time constants characteristic for the isotope and mechanism. The
total response of the ICD is a sum of prompt and delayed (or lag) terms.
For a change in neutron flux in the core, the three reactions exhibit different
response characteristic with respect to time.
The first (a) interaction discussed previously (n, β) will normally be delayed
following a change of neutron flux. The return to a new steady state
condition will be dependent upon the materials used.
Interactions (b) and (c) are prompt reactions with respect to the neutron and
gamma fluxes, respectively. Note again, however, that following a change in
fission rate a portion of the gamma signal source (due to the presence of
fission products and activation products) will exhibit a delay time before it
reaches a new steady state condition. In other words, the gamma flux at the
detector site has its own dynamic response (lag terms) relative to neutron
flux. Thus, although the Compton and photoelectric mechanisms are prompt
with respect to gamma, the current generated by it, following a change in
neutron flux, will have a prompt component from prompt gammas and a
delayed part from gamma rays emitted by fission products and activation
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products. Those products need time to reach a new equilibrium state after a
neutron flux change.
The sensitivity of the detector, that is the amount of current output per
detector unit length and per unit neutron flux, changes with exposure to
neutron flux and is known as burn-up. There are two main reasons for this:
the current producing isotopes in the detector burn up by neutron capture
and beta decay and the flux environment of the detector changes.
The sensitivity of in-core detectors to the three interactions and their
dynamic response depends on the material of the emitter.
Platinum and Platinum Clad Detectors
The response of platinum detectors with respect to the 3 mechanisms is as
follows:
•
Output due to Beta decay following neutron activation of the
platinum emitter is up to about 3%
•
Input due to Compton and photo-electrons produced by neutron
capture gamma rays (n, γ) from the emitter is around 60%.
•
Output due to Compton and photo-electrons produced by external
gamma sources is around 40%.
Output (a) is delayed with a half-life of 30 minutes, i.e., a typical time to
stabilization of this component following a neutron flux change would be
150 minutes (5 half-lives).
The dynamic response of the ICD signal shows small changes with exposure
to neutron radiation. For Pt and Pt-clad detectors the percentage declines
with neutron exposure time from about 90% in the beginning to about 80%
after very long (~ 20 years) neutron exposure.
Note that this type of ICD detects both neutron and gamma rays resulting
from the fission reaction. This feature has several advantages:
•
Prompt response.
•
Less neutron loading on reactor (low capture cross section).
•
Less temperature dependence.
•
Less susceptible to local flux perturbations.
•
Little change in sensitivity with respect to time.
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Inconel Detectors
For ICD detectors with inconel emitters the contribution of direct Compton
and photoelectric mechanisms are usually estimated at less than 10%.
Practically all the signal current comes from the (n, γ) followed by a
Compton or photoelectric electron interaction, with zero from the (n, β)
mechanism. The interesting aspect of the inconel detector dynamic response
is that it is over prompt or to put it differently, its delayed component is
negative. Thus, following a step change in neutron flux in the core the
inconel detector responds right away with about 101-104% of the
equilibrium signal. (The three main delayed components of the signal have
time constants equal to 95 sec,
25.6 min. and 3.7 hrs.) Inconel detectors are used at Darlington (RRS and
SDS1) and at Bruce B (SDS1).
Vanadium Detectors
We will mention the last category Vanadium detectors for reference only.
They have been used for flux-mapping of the core but are seldom used at the
present. Vanadium detectors are used for applications where speed of
response is not as important as accuracy of neutron flux value. For this a
detector, which is only neutron sensitive, is required. This feature is
necessary when mapping the three-dimensional flux shape throughout the
reactor. Vanadium detectors are essentially 100% neutron sensitive, i.e., the
(n, β) mechanism contributes nearly 100% to the signal. The response is
dependent upon the emission of beta particles from the emitter following
neutron activation. The response is delayed following a change in neutron
flux levels.
Factors Affecting in Core Detector Measurement
1. Fuelling or reactivity device movements nearby.
Platinum ICD signals, although very fast, do not reliably
indicate average power in the zone because they only sample
the flux in their region. They are therefore susceptible to
localized flux changes such as refuelling or reactivity
mechanism movement.
2. Start-up of the reactor.
At start-up, there is not enough neutron flux and the neutron
interaction rate is very low. Good discrimination against
gamma rays is essential in this range and it can be
accomplished by using either fission chambers or BF3
proportional counters. Also, Platinum ICDs have delayed
components, approximately 15% to 17% of the total signal
lags power change by a time constant varying from a few
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seconds to hours.
So the signal produced by the detector during a power change
may not accurately represent the true fuel power
3. Long-term exposure to neutron flux.
ICD's suffer burn up under irradiation. However, with the use
of platinum detectors, this is a relatively minor factor being
limited to approximately 1% per year.
4. Moderator poison (shielding).
In the event of poison addition to the moderator, for example
to provide an equivalent xenon load on start-up, the signal
output will be reduced due to the presence of the neutron
absorbing poison between the detector and the neutron source
(fuel).
2.5.7 Reactor Control at High Power
At the higher end of the power range the ion chambers are not sufficient for
power control. Although the ion chamber and the linear amplifier are
accurate from 10-5% to 150% full power (FP) , the leakage flux they monitor
is not a good representation of the flux in the reactor core for the simple
reason that these detectors are installed outside the reactor. The leakage flux
will be strongly affected by moderator poison, fueling in adjacent channels,
flux tilts, movements of nearby reactivity devices, etc., and it will be
relatively insensitive to power changes in the central core or the far side of
the reactor.
This is particularly important in the last decade for the following reason:
above about 20% - 25% FP CANDU reactors require spatial (also called tilt)
control capabilities in addition to the overall or bulk control function. This is
needed in order to remove neutron flux tilts caused by local reactivity
changes (due to fuelling or movements of some reactivity devices) and to
prevent xenon oscillations. To provide spatial control, the Reactor
Regulating System (RRS) needs neutron sensors distributed throughout the
reactor. Ion chambers are too big and too fragile to be put inside the reactor
core. Thus in the last power decade, the RRS switches from ion chambers to
in-core detectors for measuring and controlling bulk and spatial power.
However, in-core detectors can generate a reliable signal only from about
1% to 120% full power. Below 1% FP, the in-core detector signal is not
distinguishable from noise. Therefore, below about 10% FP ion chambers
have to be used to keep the reactor under the RRS control.
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1.5
1
3x10 14
2x10 14
10 -1
2x10 13
10 -2
2x10 12
10 -3
2x10 11
10 -4
2x10 10
10 -5
2x10 9
10 -6
2x10 8
10 -7
2x10 7
10 -8
2x10
6
10 -9
2x10 5
10 -10
2x10 4
10 -11
2x10 3
10 -12
2x10 2
10 -13
2x10
78
Power
Range
In Core Detectors
Controls at about 15% F.P.
Low
Power
Range
Ion Chambers of RRS
Controls above 5x10 -7 F.P.
10 -14
Reactor Power
(Fraction of F.P.)
Initial
Startup
Range
2
Thermal Neutron
Flux (n/cm 2 /sec)
Figure 10(a)
Neutron Detector Over-Lap
2.5.8 Overlap of Neutron Detection
At various stages in the CANDU reactor operation, the proportional
counters, uncompensated ion chambers and self-powered in-core flux
detectors are used to give a measurement of reactor power from source level
to 150% full power. A minimum overlap of one decade is provided between
successive ranges of instrumentation as schematically portrayed two
different ways; a bar graph Figure 10(a) and neutron flux vs. reactor power
Figure 10(b). The overlap of instrumentation maintains continuity of
measurement between the ranges of sensitivity.
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Running Instrumentation
(Linear In-Core Flux Detection)
0.15 - 1.5 FP
1014
Neutron
Flux
13
(n/cm2 s) 10
1012
79
C
0
20%
40%
Note
Reactor Power
60%
80%
100%
1011
1010
109
B
Runup Instrumentation
(Log Rate External Ion Chambers)
10-7 - 1.5 FP
108
107
106
Startup Instrumentation
(scintillation)
A
0 - 10-6 FP
Figure 10(b)
Neutron Detector Over-Lap
Notice again that the ion chambers generate a current signal proportional to
the reactor power level over seven decades of flux. The in core detectors, as
mentioned previously, generate a current signal over the final two decades
of flux, but are used only in the last decade.
There are several means of transferring reactor control (RRS) between the
over-lapped instrumentation:
The transfer between log and linear control in some locations is done at 17%
FP on rising power, as measured by the ion chambers and at 13% FP on
falling power, as measured by the ICD’s. This 4% hysteresis prevents
cycling between log and linear control and also allows for slight differences
in calibration due to the different types of detector used.
Another similar method employs a 13% - 17% hysteresis, but linear power
(ICD signal) is used for switchovers (in both ways) between log and linear
control to ensure a bump-less transfer.
A different method that can be used requires the ion chambers to be
thermally calibrated. The transfer is implemented between 5% and 15% FP
by progressively phasing out the ion chamber signal and phasing in the incore detector signal as power increases. Again, a bump-less transfer is
ensured.
When the RRS transfers from ion chambers to in-core detectors, there is also
another associated change, namely the power measuring and indicating scale
changes from a logarithmic (log) to a linear one.
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The log signal comes from a log signal amplifiers associated with ion
chambers as shown in Figure 5.
A log scale is useful for measurements spanning several decades because it
gives a constant signal change per decade, e.g., 2 volts per decade of neutron
power. However, it bunches up (flattens out) at the top of each decade,
which is inadequate for the last decade (see Figure 10b) where the most heat
is generated and fine control is required. For this reason in-core detectors
use a linear scale, which is more intuitive. Because of the scales, we talk
about the two RRS bulk control ranges; the logarithmic range from 10-5%
FP to about 15% FP and the linear range of control from about 15% to about
120% FP. Because of the names it is easy to forget that the really important
difference is in the neutron sensors used for control and that the scale
depends on the associated type of amplifier or a computer algorithm
transforming a linear value to a logarithmic one or vice versa.
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1.
REVIEW QUESTIONS - EQUIPMENT
Briefly explain how each of the following devices is used to measure
pressure.
Motion
Cross
Section
Pressure
a.
Bourdon Tube
Motion
Flexible
Bellows
Pressure
b.
Bellows
Motion
Flexible
Membrane
Pressure
c.
Diaphragm
Motion
Flexible
Membranes
continuous
seam
seam
Pressure
d.
Capsule
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2.
Explain how the capacitor capsule, differential pressure transmitter
detects pressure differences.
3.
Describe a strain gauge.
4.
A pressure-measuring instrument is designed around a bourdon tube.
Explain how extreme changes in the ambient temperature of the
bourdon tube will introduce errors into the readings of the
instrument.
5.
Briefly describe how the ambient pressure in a room containing a
pressure transmitter can affect the reading of the transmitter.
6.
Explain how flow can be measured using an orifice plate, venturi or
flow nozzle.
7.
Explain how elbow taps are used to measure the flow in a steam line.
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8.
9.
83
Briefly describe the how each of the following devices is used
to measure flow.
a.
Orifice plate
b.
Venturi
c.
Flow nozzle
d.
Elbow tap
e.
Annubar
f.
Pitot tube
For each of the following devices explain how flow
measurements will be affected by
a.
Changes in fluid temperature
b.
Changes in fluid pressure
c.
Erosion
i. Orifice plate
ii. Venturi
iii. Flow nozzle
iv. Elbow tap
10.
The drawing below shows a typical flow control loop. Explain
the purpose of the square root extractor.
Orifice Plate
Flow
High
Pressure
Low
Pressure
FT
4-20mA
∆P
4-20mA
Q
Controller
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11.
12.
84
A differential pressure transmitter is calibrated to measure the flow
of a liquid. Explain what will happen if the fluid is not pure liquid
but contains some vapor bubbles.
On the following drawing identify the following.
a.
Three valve manifold
b.
Primary element
c.
Transmitter
Flow
1"
H.P. Isolating Valve
L.P. Isolating Valve
Equalizer Valve
H
L
H.P. Block
L.P. Block
FT
D/P Cell
13.
The following diagram shows a density corrected flow loop. Briefly
explain the operation of the loop.
Flow
High Pressure
Sensing Line
RTD
Low
Pressure
Sensing Line
Pressure
Cell
DP Cell
Logic
4-20 mA Output
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14.
In a flow loop using a venturi for a primary element what will be the
consequences of the following abnormalities?
a.
Vapor formation in the throat
b.
Clogging of the throat by foreign material
c.
Leaks in the Hi pressure sensing line
d.
Leaks in the low pressure sensing line
15.
Sketch the typical installation of a pressure transmitter on an open
tank, measuring the level of the fluid in the tank. Explain how this
transmitter derives a level signal.
16.
Sketch the typical installation of a dry leg level transmitter
installation. Explain how this transmitter derives a level signal.
17.
Sketch the typical installation of a wet leg level transmitter
installation. Explain how this transmitter derives a level signal.
18.
The three-valve manifold on a level transmitter must be operated
correctly when either removing the transmitter for service or
returning it to service. Explain how the transmitter may be damaged
by incorrect operation of the manifold.
19.
Zero elevation and zero suppression are calibration techniques used
in level transmitter calibrations. Explain the purpose of these
techniques.
20.
Explain how a bubbler is used to measure the level in an open tank.
21.
Explain how a bubbler is used to measure the level in a closed tank.
22.
A dry leg level transmitter installation is measuring the level of a hot
water tank. What happens to the level and the level indication if the
temperature of the tank is increased and no water leaves the system?
23.
A dry leg level transmitter installation is measuring the level of a hot
water tank. What happens to the level and the level indication if the
static pressure on the tank is increased and no water leaves the
system?
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24.
86
Describe the effects on a level transmitter of the following
abnormalities in a wet leg configuration.
a.
a leak in the high pressure sensing lines
b.
a leak in the low pressure sensing line
c.
a completed block low pressure sensing line
25.
Explain how an RTD is used to measure temperature. Include in
your answer a statement to explain why there are three leads from
the temperature transmitter to the RTD.
26.
What type of signal does a thermocouple produce?
27.
RTDs are used to measure the temperature of the reactor outlet
feeders. Thermo couples are used to measure temperatures on the
turbine. Explain the reasons for the selection of these devices for
their respective applications.
28.
State the power ranges each of the following neutron detectors will
provide the signal for bulk reactor power control.
a.
Start-up instrumentation
b.
Ion Chambers
c.
In Core Detectors
29.
Why is there an overlap of the ranges over which the various neutron
detectors are used to control reactor power?
30.
Using the following diagram explain how a neutron flux signal is
derived in a BF3 detector
Cathode
BF3 gas
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31.
Explain the reason that BF3 detectors burnout if they are left installed
when the reactor is at a high power level.
32.
Briefly describe how the following ion chamber develops a signal
proportional to flux.
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37.
Explain how each of the following factors can affect the accuracy of
the in-core detector flux measurement
a. Fueling or reactivity device position
b. Start-up of the reactor
c. Long term exposure to neutron flux
d. Moderator poison
38.
State the ranges over which the in core detectors and the ion
chambers are used to control reactor power. Explain why each is
used over this range.
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CONTROL
3.0
INTRODUCTION
Control of the processes in the plant is an essential part of the plant
operation. There must be enough water in the boilers to act as a heat
sink for the reactor but there must not be water flowing out the top of
the boilers towards the turbine. The level of the boiler must be kept
within a certain range. The heat transport pressure is another critical
parameter that must be controlled. If it is too high the system will
burst, if it is too low the water will boil. Either condition impairs the
ability of the heat transport system to cool the fuel.
In this section we will look at the very basics of control. We will
examine the fundamental control building blocks of proportional,
integral and differential and their application to some simple systems.
3.1
BASIC CONTROL PRINCIPLES
Consider a typical process control system. For a particular example let
us look at an open tank, which supplies a process, say, a pump, at its
output. The tank will require a supply to maintain its level (and therefore
the pump’s positive suction head) at a fixed predetermined point. This
predetermined level is referred to as the setpoint (SP) and it is also the
controlled quantity of the system.
Clearly whilst the inflow and outflow are in mass balance, the level will
remain constant. Any difference in the relative flows will cause the level
to vary. How can we effectively control this system to a constant level?
We must first identify our variables. Obviously there could be a number
of variables in any system, the two in which we are most interested are:
The controlled variable - in our example this will be level.
The manipulated variable – the inflow or outflow from the system.
If we look more closely at our sample system (Figure 1), assuming the
level is at the setpoint, the inflow to the system and outflow are
balanced. Obviously no control action is required whilst this status quo
exists. Control action is only necessary when a difference or error exists
between the setpoint and the measured level. Depending on whether this
error is a positive or negative quantity, the appropriate control correction
will be made in an attempt to restore the process to the setpoint.
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Henceforth, the error will always take the form of:
Error = Setpoint – Measured Quantity
OR
e = SP - M
Inflow
Supply
l
ll
l
ll
l
ll
Level
(Controlled Variable)
Pump
Overflow
Figure 1
The control action will be either to vary the inflow or outflow from the
system in order to keep the level at the setpoint. Let us consider the
general format for achieving these objectives.
As can be seen from Figure 2, the process can be represented by a closed
loop. The system output (level) is monitored by a process sensor and the
measurement signal is fedback to a comparator at the input of the
system. The second input to the comparator is the setpoint signal; the
comparator’s output being the difference or error signal. The amplifier, a
present just a black box, will provide the appropriate correction to
maintain the process at its setpoint despite disturbances that may occur.
It can be seen that if the system were being operated in manual control
the feedback path would not be present. The operator would provide this
feedback and apply the necessary correction to the system whilst
observing the effect on the controlled variable. This is termed open loop
operation.
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Note
Disturbances
Controller
Comparator
Set Point
Error
(SP - M)
Measurement
Signal
Amplifier
Manipulated
Variable
Process (Tank)
Output
(Controlled
Variable - Level)
Process
Sensor
Figure 2
Feedback Control Block Diagram
3.1.1
Feedback Control
This concept justifies the use of the word negative in three ways:
•
The negative aspect of feeding the measured signal backwards
from the output to the input of the system. (Actual definition of
negative feedback control).
•
The control correction must be negative in that a correction
rather than a compounding of error must occur.
•
The fact that an error must occur before a correction can take
place, i.e., retrospective or negative control action.
In the next section we will study in more detail the methods used to
effect the necessary control corrections.
3.1.2
Feedforward Control
If we wish to control our process without an error first occurring, we
must base our control on correction of the disturbances, which will
eventually, cause a process error. This is termed feedforward control.
Feedforward control is rarely if ever used on its own but is used in
conjunction with feedback control to improve the response of control to
process disturbances.
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3.1.3
Summary
•
Controlled Variable – output quantity of system (Level,
Temperature, etc.).
•
Manipulated Variable – means of maintaining controlled variable
at the setpoint.
•
Error signal – equals the difference between the setpoint and the
measurement. (e = SP – M).
•
Setpoint – desired process level. (SP)
•
Measurement – actual process level. (M)
•
Closed Loop – automatic control.
•
Open Loop – manual control.
•
Feedback control is error correction following a disturbance.
•
Feedforward control is control of disturbances, which could
cause a process error.
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V1
SP
l
ll
Qin
Qout
3.2
l
ll
L2
L1
93
Note
Inflow
Electrically Operated
Solenoid valve
P1
Solenoid
Power Supply
P1 closes @ L1
P1 opens @ L2
ON/OFF CONTROL
Figure 3
Typical On/Off Control Scheme
Let us consider our level control system in a little more practical detail.
The valve in the inflow line to the system is an electrically operated
solenoid valve. (Remember an electrically operated solenoid valve has
only two operating positions – fully open or fully closed.) Assume that
under initial conditions with a demand on the system the level will start
to fall and V1 will have to be opened to provide an inflow. This can
easily be achieved by mounting a differential pressure switch, P1 at the
bottom of the tank to operate when the level falls to L1. When the level
is at L1 the liquid will be height h1 above switch. The pressure at the
switch will be P1 = ρgh1.
ρ – the mass density of the liquid
g – the acceleration due to gravity
h1 – the height of the liquid
The resulting switch closure can energize the solenoid valve V1 causing
an inflow to the tank. Assuming the valve is correctly sized, this will
cause a rise in the level back towards the setpoint.
In order to arrest the rise in level the built in differential feature of the
switch can be employed to de-energize the solenoid valve when level L2
is reached. This system will achieve a mean level in the tank about the
desired setpoint. This method is known as ON/OFF control. Clearly it is
impossible to maintain the system at the setpoint since there must be a
difference in the operating levels L1 and L2 as the valve can only be
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energized or de-energized. It is often counter productive to try to reduce
the differential between L1 and L2 to too small a value as this will result
in excessive cycling, and hence wear, of the valve. Usual practice is to
control with a deadband about the setpoint as shown in Figure 4.
L2
SP
time
L1
Periodic Time
Figure 4
Typical On/Off Response
The sinusoidal cycling is typical of on/off control. on/off control can be
used to advantage on a sluggish system, i.e., where the periodic time is
large. Typical uses in CANDU units are electric heater controls in deaerator tanks and PHTS bleed condenser and pressurizer.
If fine control is required a simple on/off control system is inadequate.
We will discuss a method for achieving a finer control in the next
section.
3.2.1
Summary
•
On/off control - control signal is either 0% or 100%
•
Control at setpoint not achievable, a deadband must be
incorporated.
•
Useful for large, sluggish systems particularly those
incorporating electric heaters.
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3.3
95
BASIC PROPORTIONAL CONTROL
Note
In our example of on/off control it was seen that an all or nothing control
correction was applied as the result of an error signal occurring. Clearly
it would be to our advantage if the control signal were proportional to
the magnitude of error. This is the basis of proportional control and is
the most frequently encountered control mode. How can this control be
achieved? Referring to Figure 5 it can be seen that we can modify our
system to use a pneumatically operated control valve and a level
transmitter with a 20 – 100 kPa pneumatic output.
V1
SP
Qo
l
ll
Qin
l
ll
A/C
LT
20 - 100 kPa
Figure 5: Level Control of Open Tank
If the outflow (Qo) increases then the level in the tank will fall. The
pressure sensed by the level transmitter, which is representative of the
level in the tank, will also fall causing a decrease in the output signal
from the level transmitter. This output signal is fed to the (air to close)
control valve (valve fully open with 20 kPa signal, fully closed with 100
kPa signal). A falling level will therefore cause the valve to
progressively open and hence raise the level in the tank. The system as
shown is somewhat impractical as the initial setpoint conditions will
need to be set by some manual method and then ensuring that steady
state conditions are achieved with the valve at, say 50% opening and a
level transmitter output of 60 kPa (50% range).
This simple system does illustrate however a major disadvantage with
proportional control. Notice that the control signal (valve opening) can
only change when the level signal is changing. Thus if a disturbance
occurs, say an increase in demand, the level will drop and the output
from the level transmitter will also fall. This will cause the air to close
valve to open more, hence increasing the inflow.
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After a period of time the inflow will have increased such that a now
mass balance is established between inflow and outflow. But where is
the level at this time?
Certainly not at the setpoint. In the example given it will stabilize at
some steady state level below the setpoint. This steady state deviation is
known as offset and is inherent in all proportional control systems.
Despite this obvious disadvantage, (we cannot return the process to the
setpoint after a disturbance with proportional control) this mode of
control will form the basis for all our control strategies. In the next
section we will discuss a more practical control scheme using
proportional control and also ways of lessening the problem of offset.
Example 1
A tank has inflow and outflow equal to 50% of maximum and its level is
at the setpoint, say 50%. A step change in outflow occurs to 60%
(+10%). Outflow now exceeds inflow so the level will fall. The output
from the level transmitter will also fall and, for our system, will match
the fall in level – say 1% change in signal for a 1% change in level. The
LT signal will open the A/C valve more, by 1% in fact. The inflow is
now 51%, still less than the outflow. The level will continue to fall until
inflow equals outflow, i.e., (60%). This can only happen when the LT
signal has changed by 10%) and this change reflects a drop in level on
10%: i.e., 10% offset.
To restore the process to the setpoint requires a further increase of
inflow. This increase can only be achieved by a further decrease in
signal to the valve (i.e., as decrease in LT output corresponding to a
further decrease in level).
With the conditions as stated in the example there is no way in which a
50% level can be achieved with a 60% outflow. A 50% level with a 60%
outflow requires a 60% inflow. Our systems can only provide a 60%
inflow from a 40% level signal.
Example 2
An alternative method of illustrating proportional control is by means of
a simple float system (Figure 6). Assume the inflow and outflow are
equal and the level is at the setpoint. If an increase in outflow occurs the
level in the tank must fall. The float will also fall as the level falls. This
drop in float position will cause the valve on the inflow to open more
thus increasing the inflow. Eventually the fall in level will result in a
valve opening, which will restore the mass balance between the inflow
and the outflow.
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Note an increased inflow can only be achieved as a result of a lower
level in the tank. The level is no longer at the setpoint an offset has been
generated.
Inflow
Valve
Pivot
ll
l
l
l
l
l
l
l
ll
l
l
Float
Outflow
Figure 6
Simple Proportional Control
3.3.1
Summary
•
Proportional control provides a control signal, proportional to the
magnitude and direction of the error signal.
•
After a disturbance, proportional control will provide only a new
mass balance situation. A change in control signal requires a
change in error signal, therefore offset will occur.
•
Proportional control stabilizes an error; it does not remove it.
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3.4
Proportional Control
3.4.1
Terminology
Note
M = Measurement Signal
SP = Setpoint
e = Error
e = SP – M
Note: If M>SP then e is negative
If M<SP then e is positive
m = Controller Signal Output
k = Gain
∆ in O/P = final – initial
when controller uses e = SP – M
THEN K is negative for Direct Acting
K is positive for Reverse Acting
b = bias (usually 50% of output span)
m = ke + b
↑↑ Direct Action M↑m↑
↑↓ Reverse Action M↑m↓
PB = Proportional Band
Small (narrow) PB = High Gain
Large (wide) PB = Low Gain
Gain (k) =
3.4.2
100% %∆Output
%∆valve
=
=
PB
%∆Input
%∆process
Practical Proportional Control
A more practical proportional control scheme can be achieved by
inserting a controller between the level transmitter and the control valve.
This will eliminate the setting up problems mentioned in the previous
module (i.e., it will have a setpoint control) and also introduce other
advantages, which will be discussed in this section.
In a practical system one of the primary considerations is the failure
mode of the valve.
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In our example of an open tank with a valve on the inflow it would be
reasonable to assume that the valve should close in the event of an air
supply failure to prevent the tank overflowing, i.e., an air to open valve.
A/O
SP
l
ll
Qout
Qin
l
ll
20 - 100 kPa
LIC
LT
Figure 7
Open Tank Control
To achieve the necessary control action on, say, a falling tank level it is
necessary to convert the decreasing output of the level transmitter to an
increasing input signal to the control valve. The level controller will
perform this function and is termed an indirect or reverse acting (↑↓)
controller. It can be seen that if the valve action had been chosen air to
close, then this reversal would not have been required and a direct (↑↑)
acting controller could have been used. Normally controllers are capable
of performing either control action, direct or reverse, by a simple
switching process.
The controller will also accept our desired setpoint input and perform the
comparison between setpoint and measurement to calculate the error’s
magnitude and direction.
Up to now we have only assumed proportionality constant or one, i.e.,
the control signal equals the input error. Is this always the best ratio?
Consider the following graphs of input, output and level with respect to
time:
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New mass balance
occurs here
Loss in Volume
Note
Outflow
Inflow
Input/Output
t0
time
t1
t
Level originally
at setpoint
Offset
Level
t0
t1
New level
below setpoint
time
Figure 8: Proportional Control Response Curve
It can be seen that a step increase in demand (outflow) has occurred at
time t0. the resulting control correction has caused a new mass balance to
be achieved after some time t1. At this time, under the new mass balance
conditions, the level will stabilize at some level below the original
setpoint, i.e., an offset has occurred, the loss in volume being
represented by the shaded area between the input and output curves.
New mass occurs here
Loss in
Volume
Outflow
Input/Output
Inflow
t
t
0
time
1
t
Level originally
at setpoint
Level
Offset
t
0
t
New level
below setpoint
1
time
Figure 9
Proportional Response with a lower Proportional Band
Consider now the same demand disturbance but with the control signal
increased in relative magnitude with respect to the error signal; i.e.,
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instead of control signal = error signal, control signal = error signal x
gain constant (k). Clearly for any given error signal the control signal
will be increased in magnitude, the inflow will be increased, and a new
mass balance will be achieved in a shorter time as shown in Figure 9. (If
we refer back to our simple ballcock system in section 3.3, it can be seen
that the gain could be varied by adjusting the position of the valveoperating link on the float arm.) The offset is much reduced. In
instrumentation this adjustment of controller gain is referred to as
proportional band (PB).
Proportional band is defined as that input signal span change, in percent,
which will cause a hundred percent change in output signal.
For example if an input signal span change of 100% is required to give
an output change of 100% the system is said to have a proportional band
of 100%. If the system was now adjusted such that the 100% change in
output was achieved with only a 50% change in input signal span then
the proportional band is now said to be 50%. There is a clear
relationship between proportional band and gain. Gain can be defined as
the ratio between change in output and change in input.
gain =
∆output
∆input
By inspection it can be seen that a PB of 100% is the same as a gain of
one since change of input equals change in output. PB is the reciprocal
of gain, expressed as a percentage. The general relationship is:
gain =
100%
PB
Example:
What is the gain of a controller with a PB of?
a)
40%, b)
200%
Answer:
a)
gain =
b)
gain =
100%
PB
=
100%
40%
= 2 .5
100% 100%
=
= 0 .5
PB
200%
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What will the PB setting in percent for a controller with gain of?
a)
3,
b)
Note
0.4
Answer:
100% 100%
a) PB =
=
= 33.33%
gain
3
b) PB =
100% 100%
=
= 250%
0 .4
gain
Small values of PB (high gain) are usually referred to as narrow
proportional band whilst low gain is termed wide proportional band.
Note there is no magic figure to define narrow or wide proportional
band, relative values only are applicable, for example, 15% PB is wider
than 10% PB, 150% PB is narrower than 200% PB.
Load Change
We have seen from the two earlier examples that increasing the gain,
(narrowing the PB) caused the offset to be decreased. Can this procedure
be used to reduce the offset to zero?
System Response
SP
Step Disturbance
time
"Wide" PB
Offset
SP
SP
"Moderate" PB
Offset
"Narrow" PB
time
Figure 10: Response Versus PB, Proportional Control Only
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System Response
Note
A
A/16
A/4
time
Figure 11
¼ Decay Response Curve
With reference to Figure 10, consider a high gain system (say gain = 50,
PB = 2%). Under steady state conditions with the process at the setpoint
the inflow will have a constant value. This is usually taken to be a
control signal of 50% for a proportional controller with the process at
the setpoint. In other words we have a 50% control capability. With our
high gain system it can be seen that the maximum control signal will be
achieved with an error of =1% (control signal = gain x error). This
control signal will cause the valve to go fully open, the level will rise
and the process will cross the setpoint. The error signal will now change
sign and when the error again exceeds 1% the resultant control signal
will now cause the valve to fully close hence completely stopping the
inflow. This process will be repeated continuously – we have reverted to
an on/off control situation with all the disadvantages previously
mentioned. Obviously there must be some optimum setting of PB which
is a trade off between the highly stable but sluggish low gain system
with large offset, and the fast acting, unstable on/off system with mean
offset equal to zero. The accepted optimum setting is one that causes the
process to decay in a ¼ decay method as shown in both Figures 10and
11.
The quarter decay curves show that the process returns to a steady state
condition after three cycles of damped oscillation. This optimization will
be discussed more fully in the section on controller tuning.
Recall the output of a proportional controller is equal to:
m = ke
where m = control signal
100%
k = controller gain =
PB
e = error signal = (SP – M)
Clearly if the error is zero the control signal will be zero, this is an
undesirable situation. Therefore for proportional control a constant term
or bias must be added to provide a steady state control signal when the
error is zero.
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For the purposes of this course we will assume the steady state output of
a proportional controller when at the setpoint to be 50%. The equation
for proportional control becomes:
m = ke + b
where b = bias (=50% added to output signal)
Calculation of Offset
Example:
An air to open valve on the inflow controls level in a tank. When the
process is at the setpoint the valve opening is 50%. An increase in
outflow results in the valve opening increasing to a new steady state
value of 70%. What is the resulting offset if the controller PB is:
a)
b)
50%
25%
Answer:
To achieve correct control the controller will be reverse (↑↓) acting.
a)
PB = 50% ∴gain = 2
Change in valve position = 70 – 50 = 20%
This is the output change from the controller
∆output
∆input
20%
2=
∆input
∴ ∆input = 10%
gain =
Since controller is reverse acting D measured variable must have been
negative, i.e., -10%. This is equal to a + error or a – offset. ∴offset = 10% below setpoint.
b)
PB = 25% gain = 4
∴input = 5%
offset = -5% below setpoint.
Note that the narrower PB is likely to introduce some degree of
oscillation into the system. Hopefully this will be a damped oscillation.
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3.4.3 Summary
• The controller action must be chosen (either direct ↑↑ or reverse
↑↓) to achieve the correct control response.
100%
100%
or gain =
gain
PB
•
Proportional Band =
•
The optimum settings for PB should result in the process
decaying in a ¼ decay mode.
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3.5
106
Reset of Integral Action
Note
Most of the processes we will be controlling will have a clearly defined
setpoint. If we wish to restore the process to the setpoint after a
disturbance then proportional action alone will be insufficient.
Consider again the diagram (Figure 12) showing the response of a
system under proportional control.
System Response
Step Disturbance
time
Figure 2: Additional Control Signal Restores Process to Setpoint
SP
Offset
Figure 12: Response Curve: Proportional Control Only
If we wish to restore the process to the setpoint we must increase the
inflow over and above that required to restore a mass balance. The
additional inflow must replace the lost volume and then revert to a mass
balance situation to maintain the level at the setpoint. This is shown in
Figure 13. This additional control signal must be present until the error
signal is once again zero.
Initial mass balance
Final mass balance
Outflow
Reset Action
Inflow
Setpoint
Offset
Removed
time
Figure 13
Additional Control Signal Restores Process to Setpoint
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This additional control signal is known as Reset action, it resets the
process to the setpoint. Reset action is always used in conjunction with
proportional action. Mathematically, reset action is the integration of the
error signal to zero hence the alternative nomenclature – Integral action.
The combination of proportional plus reset action is usually referred to
as PI control.
The response of PI control is best considered in open loop form, i.e., the
loop is opened just before the final control element so that the control
correction is not in fact made. This is illustrated in Figure 14.
error
Control Signal
e
t
set
ese ormal Re
R
t
N
Fas
Slow Reset
}
ke
Proportional
Response
time
Figure 14
Proportional Plus Reset, Open Loop Response
It can be seen that proportional action will be equal to ke where k is the
gain of the controller. Reset action will cause a ramping of the output
signal to provide the necessary extra control action.
After time, say t, the reset action has repeated the original proportional
response; this is the repeat time, the unit chosen for defining reset action.
It can be seen that increased reset action would increase the slope of the
reset ramp.
Note that proportional action occurs first followed by reset action.
Reset action is defined as either reset rate in repeats per minute (RPM)
or reset time in minutes per repeat (MPR).
MPR =
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RPM
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Example:
A direct acting controllerhas a proportional band of 50% ia subjected to
a sustained error. The set point is 50% amd the measurement 55%.After
4 minutes the total output signal from the controller has increased by
30%. What is the reset rate setting in RPM and MPR?
Answer:
gain =
PB = 50%
100%
=2
50%
Since ↑↑ k will be negative
Proportional Signal
= -2 x error = -2 x -5% = +10%
Total signal after 4 minutes
= +30%
=P+I
∴Integral Signal = +20%
i.e., integral action has repeated original proportional signal twice in 4
minutes, 2 repeats per 2 minutes or 0.5 repeats per minute.
Reset rate
= 0.5 RPM or
1
MPR
0 .5
= 2.0 MPR
We have already mentioned that the optimum setting for proportional
control is one, which produces a ¼ decay curve. What is the optimum
setting for reset action? We will discuss this more fully in the module on
controller tuning. For now, let us just consider a very slow reset rate and
a very fast reset rate.
A very slow reset rate will ramp the control signal up very slowly.
Eventually the process will be returned to the setpoint. The control will
be very sluggish and if the system is subjected to frequent disturbances
the process may not ever be fully restored to the setpoint!
If a very fast reset rate is used, the control signal will increase very
quickly. If we are controlling, say, a large volume tank, the level
response of the tank may lag behind the response of the controller.
The control signal will go to its limiting value (0 or 100%) and the
limiting control signal will eventually cause the process to cross the
setpoint. The error signal will now change its sign, and reset action will
also reverse direction and quickly ramp to the other extreme.
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This process will continue indefinitely, the control valve cycling, with
resulting wear and tear, from one extreme to the other. The actual
process level will cycle about the setpoint. This cycling is known as
reset windup and will occur if the process is subject to a sustained error
and a too fast reset rate. The reset rate must be decreased (reset time
increased).
The mathematical expression for P + I control becomes:
1


m = k e +
edt  + b
∫
TR


m
= control signal
e
= error signal
(e = SP – M) ∴(+ or -)
k
= controller gain
(↑↑ = −) (↑↓ = +)
TR
b
= reset time (MPR)
= bias signal
Proportional control i.e., (proper sign of gain) inputs a 180° lag into the
system (the correction must be opposite to the error). Reset action
introduces a further lag. This fact must be taken into account when
tuning the controller. (It follows proportional action). The total lag must
be increased and is now closer to 360°. (360° lag means the feedback
signal is now in phase with the input and adding to it – the system is
now unstable.) Reset action causes the loop to be less stable.
3.5.1
Summary
•
Reset action removes offset.
•
It’s units are Repeats per Minute (RPM) or Minutes per Repeat
(MPR)
•
If reset action is faster than the process can respond, Reset
Windup can occur.
•
Reset Action makes a control loop less stable.
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•
110
Do not subject process loops with reset control to sustained
errors – the control signal will be ramped to the extreme value –
reset windup will occur.
3.6 RATE OR DERIVATIVE ACTION
Consider a control system subjected to a disturbance, which causes the
error to increase in a ramped manner. Proportional control would
respond to this ramped error with a similarly ramped output signal
whose slope is proportional to the controller gain. We could reduce the
final deviation from the setpoint, i.e., the offset, and the recovery time, if
we can provide some extra control signal related to the rate of change of
the error signal. This is termed rate or derivative action and is usually
incorporated with proportional control.
Rate action is an anticipatory control, which provides a large initial
control signal to limit the final deviation. The typical open loop response
is shown in Figure 15.
It can be seen that the derivative action gives a large, immediate, control
signal, which will limit the deviation. Proportional action is then
superimposed upon this step. When the error stops changing derivative
action ceases. Note that the displayed step response unobtainable in
practice because the normal response approximates and exponential rise
and decay.
Output
Input
The rate response gives an immediate control signal, which will be equal
to what the proportional response would be after some time, say, T
minutes. Derivative units are given in minutes. These are the minutes
advance of proportional action. Derivative action is a leading control
and, therefore, tends to reduce the overall lag in the system – the system
is somewhat more stable.
Derivative ceases as
error stops changing
Proportional
Action
Derivative
time
Figure 15
Proportional and Derivative—Open Loop Response
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Mathematically proportional plus derivative (PD) control is expressed
as:
de 

m = k  e + TD  + b
dt 

m
k
TD
e
b
= controller signal
= controller gain
= derivative time
= error
= bias signal
The use of derivative control is limited. At first glance, derivative
control looks attractive. It should help reduce the time required to
stabilize an error. However, it will not remove offset. The control signal
from derivative action ceases when the error stops changing, which will
not necessarily be at the setpoint.
Its use, in practice, is also limited to slow acting processes. If used on a
fast acting process, such as flow, control signals due to derivative action
will often drive the control valve to extremes following quite small but
de
steep (large ) changes in input.
dt
Consider a simple flow control system, consisting of an orifice plate
with flow transmitter and square root extractor plus direct acting
controller and air to close valve (refer to Figure 16). This system is
subjected to a small, but fast, process disturbance. How will this control
scheme perform under proportional and derivative control modes?
_l l
l
√ FT
A/C
FC
Figure 16
Simple Flow Control System
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To answer this question, let us consider the PD response to a fast change
in process signal in an open loop system (Figure 17).
Process
B
Control
Signal
%
A
C
t0
t1 t2
time
Proportional action
A-B
Rate action due to cessation of increase in e
Rate
Rate action
action
B -C
A-B
Control signal at end of excursion
Rate action due to cessation of increase in e
Proportional
action B - C
t0
t1 t2
time
Figure 17: The open Loop Response of Proportional Plus Derivative (PD)
Action to Rapidly Changing Error Signals
The upper portion of Figure 17 shows a positive process excursion, AB,
from the zero error condition, followed by an equal negative excursion,
BC, which returns the error to zero. Note that the rate of change, i.e., the
slope of the process change, from B to C is twice the rate of change of
the process, from A to B. Mathematically:
de
(B − C ) = 2 de ( A − B )
dt
dt
The proportional control action from B to C will be equal but opposite to
the proportional control action from A to B. The rate or derivative
control action from B to C will be double that from A to B. The resulting
open loop control signal pattern is shown in the lower portion of Figure
17. The controller gain and derivative settings remain constant.
Very shortly after time (t0) the control signal increases abruptly to a
value determined by the rate of change of the error (e), the derivative or
rate time setting, and the controller gain. Proportional action ramps the
control signal up, until time (t1), to a value determined by the error (e)
and the controller gain setting. This includes the direction of the error
and controller action.
At time (t1) the rate of change of the process error, de/dt, momentarily
becomes zero, so the original change in the control signal due to the rate
action drops out. Then, the process error change direction becomes
negative, and the derivative control action now produces an abrupt
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negative control signal, double the original derivative control signal. The
proportional control action then ramps the control signal down until time
(t2).
At time (t2) the rate of change of the process error becomes zero, so the
derivative control signal again drops out leaving the control signal at its
original bias (zero) error value. Note that this final bias, (zero) error
value of the control signal and, hence, the control valve position at the
end of this excursion, is determined solely by the proportional. The
valve has been stroked rapidly and repeated by the derivative action
subjecting it to unnecessary wear, with no improvement in control.
The response of the closed loop shown in Figure 16 would be somewhat
different because the resulting valve action would continuously alter the
error signal. However, the valve would still be subjected to rapid and
repeated stroking unnecessarily.
Thus, it can be seen from the above discussion that the use of derivative
action on fast acting processes such as flow is not advisable.
Let us look at the control of a sluggish (generally a physically large)
system. As an example, consider a large tank with a variable outflow
and a control valve on the inflow. A large volume change will, therefore,
be necessary before any appreciable change in level occurs.
Consider a large change in the outflow. After some delay (due to the
sluggishness of the system) the controller will respond.
If we have only proportional mode on the controller the delays will
mean that the controller is always chasing the error initiated by the
outflow disturbance. The response to proportional control is shown in
Figure 18. Note that the process has not fully stabilized after a
considerable period of time.
The addition of derivative action, however, causes an anticipatory
response. The control signal increases more rapidly and the process is
returned to a steady state in a much shorter time. Note also that:
The system is more stable (less cycling) with PD control.
Offset still exists.
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Control Signal
Note
Load
Disturbance
Applied
Prop. + Derivative
Prop. Only
time
Setpoint
Level
Figure 18
Large System Under Proportional and Proportional Plus Derivative
Control
3.6.1
Summary
•
Derivative or rate action is anticipatory and will usually reduce,
but not eliminate, offset.
•
Its units are minutes (advance of proportional action).
•
It tends to reduce lag in a control loop.
•
Its use is generally limited to slow acting processes.
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3.7
MULTIPLE CONTROL MODES
We have already discussed some of the possible combinations of control
modes. These are:
Proportional only,
Proportional plus reset (integral) P + I,
Proportional plus derivative (rate) P + D.
It is also possible to use a combination of all three-control modes,
Proportional plus Integral plus Derivative (P + I + D).
At a glance proportional only does not appear very attractive – we will
get an offset as the result of a disturbance and invariably we wish to
control to a fixed setpoint.
An application of proportional only control in a CANDU system is in
the liquid zone level control system. The reason that straight
proportional control can be used here is that the controlled variable is
not level but neutron flux. The manipulated variable is the water level;
therefore offset is not important as the level is manipulated to provide
the required neutron flux.
In general it can be said that the vast majority of control systems
(probably greater than 90%) will incorporate proportional plus integral
modes. (We usually want to control to a fixed setpoint.) Flow control
systems will invariably have P + I control.
Derivative control will generally be limited to large sluggish systems
with long inherent control time delays, (for example, that shown in
Figure 18.). A good general example is the heat exchanger. The thermal
interchange process is often slow and the temperature sensor is usually
installed in a thermal well, which further slows the control signal
response. Frequently heat exchanger temperature controllers will
incorporate three-mode control (P + I + D).
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3.8
TYPICAL NEGATIVE FEEDBACK CONTROL SCHEMES
3.8.1
Level Control
117
In general we can divide level measurement into three types:
Open Tanks
Closed Tanks
Bubbler Systems (Open or Closed Tanks)
If a differential pressure transmitter is used as a level detector, the lowpressure port will be vented to atmosphere in an open tank application. In a
closed tank, where there is often a gas phase at pressure above the liquid, the
low-pressure port will be taken to the top of the tank. Any gas pressure will
then be equally sensed by the high and low sides and thus cancelled.
Remember the closed tank installation will have either a wet or dry leg on
the low-pressure sides.
Open Tank Installation
Assuming the control valve is on the inflow, the best failure mode for the
valve would be to fail closed, i.e., Air to Open (A/O) valve. The pressure
sensed at the base of the tank on a falling level will decrease, i.e., controller
input. The valve must open more, to replenish the tank, requiring an
increasing signal. The controller must be reverse acting and will usually
have P + I modes. The system is shown in Figure 19
If it is necessary to mount the valve in the outflow, the best failure mode
would probably be to fail open (A/C). This valve action would require an
increasing signal to halt a falling tank level, again a reverse acting (P + I)
controller is necessary.
The same reasoning would apply to closed tank or bubbler systems, the only
difference being in the sensing method employed. Remember control modes
use of derivative action on large, slow, systems.
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Note
A/O
LIC
LT
l
ll
Qi
SP
Qo
l
ll
Figure 19
Open Tank Level Control
3.8.2 Flow Control
A typical flow control system requires some form of restriction to provide a
pressure differential proportional to flow (e.g. orifice plate) plus a square
root extractor to provide a linear signal. The controller action depends upon
the choice of control valve. If an air to open valve is chosen then controller
action should be reverse, as an increase in flow must be countered by a
decrease in valve opening. For an air to close valve the action must of
course be direct. The general format is shown in Figure 20.
_l l
√ FT
A/O
FIC
SP
Figure 20
Typical Flow Control
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The control modes will be proportional plus integral (never use derivative
on a flow control loop).
3.8.3 Pressure Control
The control of pressure in, say, a pressure vessel, is generally achieved in
one of three ways.
1. Variable Feed with Constant Bleed
2. Constant Feed with Variable Bleed
3. Variable Feed and Bleed
Consider first Variable Feed and Constant Bleed (Figure 21).
The feed valve action is air to close (A/C). Increasing pressure will
require an increasing valve signal to throttle the supply. The (P + I)
controller is direct acting. For a variable bleed application the control valve
will be transferred to the bleed application the control valve will be
transferred to the bleed line and will need to be A/O if a direct acting
controller is used.
SP
PIC
Feed
PT
Pressure Vessel
A/C
Figure 21
Pressure Control – Constant Bleed
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Pressure
Bleed
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For variable feed and bleed we can use a split range control scheme (one
controller driving two valves). This is shown in Figure 22. When at the
setpoint we require feed to equal bleed. If pressure increases we require less
feed action and more bleed action and vice versa. The valve actions must
therefore be opposite, say feed valve A/C and bleed valve A/O. On
increasing pressure the direct acting controller will supply a larger signal to
the feed valve (closing it) and to the bleed valve (opening it). Pressure
should thus be maintained at the setpoint with proportional plus integral
control.
SP
PIC
PT
A/O
Feed
Pressure Vessel
Bleed
A/C
Figure 22
Split Ranged Feed and Bleed Pressure Control
3.8.4 Temperature Control
The general problem with temperature control is the slowness of response.
For this reason the use of derivative action is fairly standard. Figure
23shows a representative heat exchanger, which cools hot bleed with cold
service water.
The choice of control valve would probably be air to close, i.e., fail open, to
give maximum cooling in the event of a air supply failure to the valve.
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Hot Bleed
Note
Cold
A/C
TT
Cooled Bleed
TC
SP
Figure 23
Temperature Control of a Heat Exchanger
An increase, say, in bleed temperature requires a larger valve opening, i.e.,
smaller valve signal. A reverse acting controller is required. Three mode, P
+ I + D, control is fairly usual.
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REVIEW QUESTIONS - CONTROL
1.
Consider a system for heating a room with electric heaters; what are
the controlled and manipulated variables.
2.
Sketch and label a block diagram of simple process under negative
feedback control. Mark setpoint, measurement, error, output,
disturbances.
3.
State the three important characteristics of negative feedback control.
4.
State the differences between feedback and feedforward control.
5.
Is driving a car (in a reasonably normal manner) an example of
feedback or feedforward control? Explain.
6.
Explain the operation of a process under negative feedback on/off
control.
7.
Why will on/off control cause cycling about the desired setpoint?
8.
Why is on/off control frequently used in room heating applications?
9.
If in figure 5, we located our control valve in the outflow line, what
would be the required valve action for negative feedback
proportional control?
10.
Explain the relationship between error and controller output in a
proportional controller.
11.
Why does offset occur with proportional control?
12.
A control scheme consists of an open tank with an air to close valve
on the outflow. Sketch a simple schematic diagram showing the
controller action. What would happen to the control of the system if
the valve was changed to air to open but the controller action was
unchanged?
13.
Why can offset not be removed by narrowing the proportional band?
14.
What gain is represented by a Proportional Band of 200%, 75%,
400%, 20%?
15.
A disturbance causes a process to change by 5%. What will be the
change in controller output if the PB is 100%, 50%, 200%?
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16.
A tank is controlled by an air to close valve on its inflow. When at
the setpoint the valve opening is 50% an outflow disturbance causes
the valve opening to become 80%. The controller’s PB setting is
50%. What is the offset (%)? Assume a linear valve characteristic.
Remember an air to close valve requires a decrease in signal to open
it further.
17.
Sketch and describe the curve which would, in many processes, be
the optimum process response following a disturbance.
18.
What is the purpose of reset action?
19.
What are the units for reset action?
20.
What is reset windup?
21.
Does reset action make the loop more or less stable?
22.
Draw an open loop curve showing the response of a proportional
plus reset control system to a step disturbance.
23.
A control system with a direct action controller is operating at the
setpoint. The controller proportional band is set at 50%. The system
is subjected to a disturbance, which creates a positive step error of
+6%. The total control output change after 18 minutes is 48%. What
is the reset setting in MPR?
24.
Using the same control system and control settings as in Question
23, what would be the effect on the system if it had been subjected to
a disturbance which caused a step error of -8% for a period of 18
minutes?
25.
What is the purpose of rate control?
26.
What are the units of rate control?
27.
Why should rate control not be used on a fast acting process such as
flow?
28.
Will rate action remove offset?
29.
What is the effect on the rate signal if the error stops changing?
30.
Which control setting gives the largest rate signal, 1 minute or 5
minutes? Why?
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31.
Sketch an open loop response graph for a proportional plus
derivative control system subjected to a ramped error signal.
32.
A proportional plus derivative control system is subjected to a
ramped error of -10% per minute for 1.5 minutes. The PB setting is
100% and the derivative setting is 3 minutes. The controller is
reverse acting. Sketch an open loop response curve for the system
showing control signal values at 10% intervals, with respect to time.
33.
Give a typical control example where straight proportional control
can be used.
34.
What is the most commonly encountered combination of control
modes and why?
35.
Why is it advantageous to use derivative action in the temperature
control of a heat exchanger?
36.
Sketch a level control scheme for an open tank. The valve selected is
A/C and on the inflow line. State controller action and modes.
37.
A heat exchanger (cooling hot bleed with cold service water) is
controlled by an air to open valve on the service water line. Sketch
the circuit showing controller action. What control modes would be
used and why?
38.
Sketch a simple electronic control scheme for the control of flow.
The valve chosen is air to close; an orifice plate develops the
differential pressure. Show controller action and state the most likely
control modes.
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